1-3 star servants

1. Having merlin does lower the priority of leveling David a bit, but you should still level him when able since you never know if Merlin's skills are down and can just order change him in.
2. Chloe is better than both in many cases, so if you only need to level one I would go with euryale for that anti male niche over poison niche.
3. Kintoki is a couples months and if you think you can last the long without a rider then yeah wait for him.
4.Medea has buff remove so she's a good choice. Cu for a good 3 star lancer. Caesar or Bedi for team support. A lot of the other good 3 or lower star servants really only have a good farming niche.

David could act as insurance if you're lacking on cost since Tristan is 4 star(9 point cost). Furthermore David survivality is much higher than Tristan due to 2 healing skill and an evade. Charisma is straight forward skill. For crit and damage Tristan can generate more star than David but old harem master David utility is higher than failed Milf chaser Tristan. Tristan can remove enemy buff while David NP has skill seal.

2. NP damage wise - Robin will deal more damage to poison enemy. Euryale will kill male berserker and saber more faster than kuro, but kuro can use her NP faster than those 2.

3. In case if you don't have him or a single target 5 star rider, just raise Yoshitsune as safety. No need to raise her skill high enough 4/4/4 or 6/6/6 is good enough until you get the Ovum no raida

4. Shakespeare is one turn buster support similar to Mozart. Leonadis and Benkei(he needs 2nd ascension) can taunt. Matahari has charm, double def down and skill seal. Kotaro has single target evade, atk down, and NP with chance of skill seal. Mephistopheles has a rare buff block debuff which can be utilize to block enemy buff after you break their HP bar. He will become quite useful.

It is always worth to have a lower star class equivalent.

1. Yes! Level David.

2. Yes again. Robin gets a lot of event bonuses and his NP damage is better. Most events don't require NP spamming so Kuro's advantages are kind of moot there. (She's an overall better CQ pick though.) Euryale's anti-male niche is very relevant.

3. Ushi is great but Ridetoki is coming in a few months. If you need an ST rider and you're sick of borrowing Ozy, level her. Kintoki outclasses her in every way but she's a solid pick until he comes in May/June. She also gets a lot of event bonuses so might be worth raising just for those.

4. Tons---

- Medea for buff clear/debuff clear + targeted golden rule.
- A lot of people swear by Cú, but I've never had to use him.
- Shakespeare's battery after rank-up can be useful for farming.
- Bedi's NP damage is amazing if you manage to NP5 him.
- Caesar is a good ST saber although Brave Liz totally outclasses him. You'll get her this Halloween if you missed her.
- Lu Bu is a budget Kintoki (but less effective against break bars).
- Spartacus and Bloodaxe are also great farmers for clearing hands/doors...
- Alexander and Medusa are also great door clearers. Alexander also has a quick support niche...run him with Okita, Billy, or Atalanta. Medusa's NP battery can be a nice quality of life boost for farming. Santa Alter definitely outclasses both in terms of raw damage...but I'd save her for events and use the 3*s (and Blackbard) for clearing doors.
- Asterios can cheese some quests against bosses with high hit counts.
- Jaguar Warrior is a burstier Proto Cú, minus survivability. Lancer Alter outclasses both, though.
- Mata Hari's debuffs can be useful against certain high-HP bosses...but she really loses effectiveness against break bars since setting up a nuke is her best use.
- Paracelsus's future rank up turns his guts into a targetable Golden Rule with values better than Bride's. Great for NP loop farming with Surfer Mo, Dantès, or Hokusai when she comes next year.
- Fergus is a good AOE saber farmer if you need one.
- Billy has some of the hardest crits in the game. If you like quick teams, he's your guy.
- Kid Gil and Tawara are AOE archer farmers if you need them.
- Hektor is a good AOE lancer farmer if you missed out on Jeanne d'Arc Alter Santa Lily.
- Blackbeard is a mini version of Drake. Can also clear doors. But Santa Alter is better.
- Babbage is a farming Caster...if you ever need one of those. Bring him for lantern farming. (He's also much underloved, imo.) Caster Liz totally outclasses him but she's not coming back if you missed her.
- Mephisto's NP damage is pretty bad but his 3-buff block on his final skill saved me from Goetia. Not much use beyond that, unfortunately. He needs a buff.
- Kiyo and Darius are AOE berserker farmers but neither of them is very food. Darius has a higher attack but Kiyo has self-buffs...I think Spartacus and Bloodaxe are better (and require less of an investment).
- Boudica is useful in budget stall teams. User her NP and Mash's NP at the same time and you'll be safe from almost anything.
- Jekyll isn't particularly useful but he is fun to play with.
- Fuuma is totally outclassed by Summer Scathach...and AOE assassins just generally aren't useful unless you're Cleopatra. He's a budget Jack the Ripper, has great stargen, and gets a lot of event bonuses.
- CA Hassan is like a mini Fuuma...stargen isn't as good but lives longer and crits harder. (Although his crits still don't do that much.) Final ascension arts is super cool, so that's a plus.
- Serenity is probably the best 3* ST assassin if only because she slots into Arts teams nicely and has decent stargen. She's like a mini Shuten. Pairs super will with Robin and gets 2 buffs this year to solidify her niche as a semi-staff/debuff heavy arts assassin. Run her on a weird DoT Arts Crit comp-except-for-nuke-turns like Merlin/Robin/Serenity. One time she instadeathed a giant dragon for me, so that was nice too. Welfare Shiki is better but she won't be back until 2020.
- Hundred personas can solo NP loop but it's very RNG-dependent. Her golden rule variant is very good though. Again, Welfare Shiki is better.

I'd recommend you raise:
- David
- Robin
- Euryale
- Ushi
- Medea
- Cú
- Bedi
- Spartacus
- Bloodaxe
- Shakespeare
- Alexander/Medusa/Blackbeard (for doors)
- And whichever of the above niches your 4* and 5* servants don't cover. So don't raise Fergus if you have Mordred, don't raise Lu Bu if you have Herc, don't raise Kid Gil if you have Emiya, etc.

Re:skills, Arash's NP battery is totally worth a lore 10. That 30% charge will come in handy during events when you can only run a 50% starting NP CE. Leave his others at 1.

Spartacus's guts/NP battery is also worth a lore. I'd bring his buster buff to 9 if you don't want to spend a lore on it because it noticeably increases his performance.

All of Euryale's skills are great and they really impact her performance but as a 3* she's harder to raise.

Alexander's first and third skills are great farming buffs and Medusa's atk buff is also good. My Alexander's skills are only at 6 and he performs great.

Ushi doesn't really need her skills and doesn't have a great ROI on leveling them but if you find yourself using her in events a lot then they're worth it.

I grailed and maxed my Robin...but he's not really skill-dependent. The poison might miss (especially against enemies with magic resistance like Mordred) so leveling Sabotage is useful.

I'd also say---look at the upcoming lotto events and see who the event bonus servants are. (And see what the best nodes require you to have.) My servants grew SO MUCH during the Christmas lotto and I only opened 30 boxes because it took me ~7 turns to clear the best node.

For Nero Fest this year, it looks like AOE riders (Alexander/Medusa/Blackbeard/Santa Alter), AOE berserkers (Spartacus, Bloodaxe, Kiyo, Darius, Cha Cha when she comes in April) ST Berserkers/anti-Male nukers (Lu Bu, Euryale, also welfare Nobu), and AOE lancers (Santa Lily, Hektor) will be the most useful for farming the lotto currency. So make sure they're raised by September.

For Christmas, Ushi, Paracelsus, Taiga, Bekei, and Leonidas get event bonuses. Benkei and Leonidas probably won't see battle so I'd just raise Ushi, Paracelsus, and Taiga. Welfare Santas also get event bonuses so definitely have them leveled.

It looks like ST and AOE archers are best for Christmas this year. David, Euryale, Robin, Kuro, Emiya, Kid Gil, Touta, ARASH, and your Tristan will be great. I'd bring Kuro and Robin because you'll need to nuke two high-HP sabers on separate waves.

And don't forget to fou everybody!

I think you need to lvl up all your servants. Even more if they OP. This way you'll have more options when you need to make an strong team to harder content.

1- Yes, David skill seal in NP and Caster deck to ease arts chains is not bad.

2- Both. Robin NP on overcharge is a nuke and his evade gives 10 to 20 free stars, Euryale has lower nuking capacity and only for males, but can stall males like nobody else

3- Ridertoki is better in all aspects, but she can be useful for event bonuses and having lower cost when you're pressed against cost...she has an evade to survive NP firing, mind you

4- Shakespeare, the Buster amadeus, Blackbeard for budget Drake and NP dmg/female heal support, Caesar if you don't have golden ST Sabers (and even if that the case, you still can rely on him for dmg support), Alexander for Quick support, Mash post Camelot in invaluable good, Kiyo to farm hands, Kotarou if you dont have Assassin Scath, Romulus/Hektor if you dont have JalterLily for AOE Lancer, Paracelsus for Arts support (Casgil is better), and NP spam support.