about stalling and team in fgo

Normally art team is the best staller due to their NP and skill set. Basic one is total defense team like Mash, Hans and Jeanne. Tamamo is art support that work well in stall team too as her NP reduce skill CD while charging around 25 - 40 % NP as usually her NP is last to use. Below is several known stall team

Mash, Jeanne, Tamamo - this is universal art support team. mash for def up and single target deflection with her NP and skills. Jeanne for HP regen and invincibility. Tamamo for healing and CD reduction
Mash, Boudica, Tamamo - double defense up from their NP can reduce damage taken to zero
Mash, Waver, Tamamo, - the def up curse team. damage usually from waver
Mash, Boudica, Hans - Someone manage to stall 400 turn using lvl 1 character on these 3
Jeanne, Waver, Hans - curse team with healing capability
Waver, Merlin, Asterios - someone manage to stall for 100 turns against Kiara(beast 3) - this team use curse damage from asterios and Waver NP while def down and atk down from asterios NP lowering enemy damage. Merlin act as healer and NP up time

Offensive Stall Team
Orion, Waver, Tamamo - these team basically keep enemy unable to fire their NP while Orion can lower enemy damage with his NP. Tamamo as healer
Stheno, Euryale, Euryale - charm lock team there's a few video on youtube on these. you can check them stall against Gawain and Hijikata for reference

My Best stall crit dps team
Tamamo+caster gill+arturia archer
This team is stupidly strong because the synegry of cas gill and arturia archer skill to make star even with arts chain and tamamo for reducing their cooldown time

Some basic of stalling in FGO is hard defense, Def Up buff stacking, HP stat, NP gain, team consistencies and tons of tricks from the player itself.

- Art stall team is the most resilient team in FGO yet and you'll find tons of them, it's their best job to stall and NP spam for the whole battle.
- For Quick teams, they're also tough and can survive longer battle with some stun or hard defense.
- For Buster team, they're not suited to this job. Swift and fast kill is the only things they can do. Only few Buster servants can do this.

Team comps, solo and stall :
Art : Kira Kira Sea Turtle, Marie Caster + Marie Rider + Marie Caster.
Quick : Kira Kira Land Tortoise, Marie Rider + Marie Caster + Marie Rider.
Buster,solo: This is Sparta, Leonidas + lvl 1 Georgios + friend Leonidas lvl 1vs high hit count Archer/enemies.
Quick : immortal doggo, Proto Chu + OG Chu + Caster Chu.
Quick, solo : Whassup Homies, Marie Rider ( MLB/non-MLB Holy Shroud of Magdalene or MLB/non-MLB Steel Training, Holy Night Supper, Golden Sumo, NP gain, NP Up, Guts ) + lvl 1 Georgios + lvl 1 Leonidas vs male Caster or berserker.
Art, solo : Unbroken Vitality, OG Nero ( Guts, MLB Maid in Halloween, NP gain, Def Up, Invincibility CEs ) + lvl 1 Leonidas + lvl 1 Georgios.
Quick, solo : The Immortal Dog, OG Chu ( Invincibility CEs, NP gain, NP Up, Quick UP, Guts, Def Up ) + lvl 1 Leonidas + lvl 1 Georgios.
Buster, solo : Sekai De Ichiban Tsuyoi Dakara, Herc ( Bond CEs ) + lvl 1 Leonidas + lvl 1 Georgios.
Buster, solo : Rampaging Wild Dog, Chu Alter ( MLB Golden Sumo, MLB Holy Night Supper, Guts, Invincibility CEs, Buster Up, NP Gain, NP Up, Guts, ) + lvl 1 Leonidas + lvl 1 Georgios.
Buster, solo : The Grim Reaper, King Hassan ( Guts, Def UP, Buster Up, NP Up, MLB Golden Sumo, MLB Holy Night Supper, Invincibility CEs ) + lvl 1 Leonidas + lvl 1 Georgios.
Art,Buster : Iron Wall of Defense, Asterios + Asterios + Darius III vs AOE high hit count enemy/enemies.
Art, Buster, Quick : The Toughest Soloist, OG Nero + OG Chu + Herc ( Bond CEs ).
Art : Walking Fortress of Camelot, Mashu + Mashu + Hans/Jeanne/Tamamo/Merlin.
Quick : Evading Some Stuff, Cursed Arm + OG Chu + Proto Chu.
Quick, Buster, Art : Cursed eyes of the Snake, Medusa Loli + Euryale + Stheno.
Art, Quick, Buster : All Star Tank, Scathach Assassin + Vlad III Lancer + Mashu + D'eon + Leonidas + Georgios.
Art : Raid Boss Saber, D'eon ( non-MLB/MLB Ideal Holy King ) + Mashu ( Ideal Holy King ) + Merlin ( Ideal Holy King ) + backline Art servants equipped with MLB Ideal Holy King.
Art : Anti-Beast III Kiara, Nightingale + Waver/Merlin/Asterios + Merlin.
Art : Unbreakable Squirt Gun, Squirtoria + Tamamo + Mashu.
Art : The Strongest, Waver + Merlin + Tamamo.
Art : Classic Stall, Mashu + Hans + Waver/Merlin/Tamamo/Jeanne.
Art : Unkillable Dragon Slayer, Georgios + Mashu + Merlin.
Art, Buster solo : The Mad Bull, Asterios ( NP gain, Guts, debuff immunity, Def Up, Invincibility CEs ) + lvl 1 Leonidas + lvl 1 Georgios vs high hit count enemy/enemies.
Art : Invincible Art Teams, Vlad III + Waver + Tamamo.

I can post more team comps but it will make my post into a longpost.

Bonus : to make things easier, here's some stuff that you'll need :
- Def Up, Damage Cut, Atk Down, Evade, Invincibility, Special Def up, Crit chance down, Crit damage down, Cards effectiveness down ( Art, Buster, Quick Card Down ).
- Heal, Heal effectiveness Up, HP Regeneration.
- enemy hit count, skill CD and stage mechanics and tons of another good stuff.

Another Bonus : Invincibility and Evade buff is stackable, however Invincibility for a turn and one turn Evade buff is stackable but it's quite useless.
Non-turn based Invincibility + Evade > turn based Invincibility + Evade.
Non-turn based Evade + Turn based Invincibility is stackable.

Invincibility will took all of the enemies atk first, non-turn Evade won't be disturbed.
3 hits Evade + 3 hits Invincibility = 6 hits hard defense buff, you won't be damaged untill all of the hard defense ''barrier'' is gone or it gets pierced by Ignore Invincibility buff. The strongest yet, buff removal work for both Offensive and Defensive buff or any survivability kit except for equipped Guts from CEs.
Ignore Evasion won't pierce Invincibility.

Stall usually has 4 components:

• Enemy NP neutralization: Usually through teamwide evasion/invincibility, but sometimes through debuffs like Asterios or Kid Gil. Tauntes like Mash and d’Eon can single handedly block ST NP with their invincibility/dodge skill...they’ll appreciate a Tamamo to reduce the cooldown on those skills.

• Enemy NP delay: Tamamo’s Curse and Jeanne’s stun are helpful in delaying an NP for a turn. Serenity’s Infiltration and NP seal on Zabaniya are some of the mean reasons to field her in a stall team. Delaying NPs means you can get Jeanne’s NP ready to block all enemy NPs on the same turn.

• Healing: Usually from skills or NPs like Tamamo, Merlin, Jeanne, Medea Lily, Nightengale, Hans, etc.

• Damage Down: Mash, Jeanne, Waver, and Hans (and CasGil, to an extent) have valuable teamwide defense up buffs to reduce the card damage you take. The less damage you take, the more effective your heals. Robin Hood’s poison applies a 15% damage down to enemies, which is the same as giving your team a 15% def buff.

So premier members are:

• Mash
• Jeanne
• Tamamo
• Waver
• Hans
• Georgios
• d’Eon
• Asterios
• Merlin
• Boudica
• Martha
• Medea Lily
• Serenity

Some team comps + hybrids:
• Mash-Tamamo-Jeanne: OG Stall Team. Will only get better after Jeanne’s interlude. Deal no damage, take no damage, everybody wins. You can sub Mash out for Waver, if you have him.

• Tamamo-Jeanne/Mash-Robin/Orion/Squirtoria: Tamamo is Tamamo. Jeanne provides stars and invincibility, but Mash can work too for dodging NPs, albeit at the cost of stargen and a stun for N atk buff on her NP. Robin has a dodge and his AOE Attack debuff, plus he plays nicely with Tamamo. Orion and Squirt each have NP drain components to their kit, so they’re useful for dealing lots of damage and delaying NPs. Squirt also has a heal.

• Asterios/Herc/CuAlter-d’Eon-Mash: With d’Eon’s 3-turn taunt and Mash’s extra taunt, invincibility, and Herc/Cú Alter’s dodge, you can actually protect your berserker 100% of the time. Asterios will likely take a hit, but his powerful rebuffing NP means he probably won’t take any damage.

• Hans-Ozy-Tamamo: With Ozy’s Protection of the Sun King, Hans gets a LOT more reliable. Tamamo has the arts cards to quickly charge your NP gauges. Ozy is a great hybrid, since he takes hits like a ruler half the time (or more frequently, with Tamamo), has a heal, has NP seal on his own NP, and naturally draws the stars from Hans + appreciates the crit steroid. This team has no way to dodge NPs (though you will be stalling against them pretty frequently) so you could use Mash or Jeanne instead of Hans or Tamamo, but you’ll miss the utility.

• Medea Lily/Tamamo-Martha-Jeanne: Unlimited heal works. Martha can also deal damage and clean Jeanne’s stun debuff. After Jeanne’s interlude this comp gets obselete, though.

• Robin-Serenity-Waver: Poison/curse comp. later this year Serenity gets 2 strengthenings where her poison damage goes to insane numbers. Robin’s poison stacks and Waver is Waver...he provides everything except heals. (Stick a healing CE on everyone.) A Waver/Serenity/Robin NP chain and also burst down your opponents and Robin and Serenity have stars to quickly charge you own NPs.

Typically hybrid teams perform better than pure stall, in my experience.

Okay so other people already provided excellent answers. Here is several my auxiliary opinions, particularly drawback:

Stall team is probably the cheapest budget team in FGO since no matter what, you will have our best Mashu. It doenst means that stall team = Mashu since there are various kind of stall team existed, but she is off course one representative. Also there are many other 1-4 star servants could be used in stall team. Like the famous Cu stall, or double Mashu-Hans, Double Geogies facing enemy caster, etc

However, every team in FGO has drawback. When you using stall team, it is strongly recommend to be aware what your enemy is. If your enemy is quite famous for buff removal, stun/charm your team member, or high hp Rider/Berserker (Drake for instance) and Amakusa, you may have to change your stall team configuration or even forbidden using stall team. Sometimes checking wiki to get familiar to your enemy is really important for using stall team in a challenge quest, although it may face spoil issue.

Basically a stall team is a team meant to prolong the battle.
how
By Delaying the enemy np, reducing damage, healing, and so on and so on.
So instead of focusing on killing the boss, it focus on keeping up the team in longer fight.
There are a lot of other guy who actually explained you many of the more amazing team (double hans and mash? i want to try it!!!), but i think we are missing something in the answer
What you need is a team able to cripple the enemy so much thet he is not going to damage you.
In order to do so, you need to form a team with skill such as def up, attack down, inv, evade, np drain, stun, heal, np seal.
those are the base skill.

there are 2 mayor way of doing this.
Buy stucking Stun and drain.
this work for enemy you fear the np. Basically you will delay the enemy np as much as possible.
For exemple, euriale have 2 charm and 1 np drain.
if you pair her with 2 other servant with a similar kit, you may be able to stunlock the boss.

the second way is the 0 damage way.
Basically you need to incrase your defence and decrase enemy attack to 0
this work becouse attack down and def up are calculated on the total of the damage.
So, even if you were to take 100000 damage, if you have a 30% attack down and a 70% def up you would take no damage.
this kind of team use heal and inv/ evade to cover the downtime of those def up skill.
Mash is a perfect fit for this kind of team.
This team work better on long battle with different boss, like this valentine challange.

now, after you have chose what kind of stall you want you have to chose the dps servant.
i know many of those team are only for pure stalling, but reaching 100 turn in a fight (is cool, i wanna try it!) is not a efficent way of doing it.
So most of the stall team have 2 support and 1 dps.
The dps need to have some stall usage to obviously.
For exemple, in the first team, euryale is perfect
Pair her with 2 support for a np drain / charm team, and she can awsome.
For the second team, a good exemple can be Cu Alter. he have def up and enemy attack down, and another def up in the np.
Put him with mash and merlin/weaver and you have your def stall team.

This is stall.