Gawain Help

Have you leveled up Euryale and David? They're easily the best choices for this fight.

Euryale can charm Gawain and can drain his NP. David has a full party Evade to help the party survive Gawain's NP if you can't stop it.

Euryale is Gawain's worst nightmare. She makes this fight pretty easy imho.

I personally used her, Robin and a support Orion. Kept stalling his NP with 2 charms and 2 drains. By the time the frontline got wiped out he had only 30k hp left and was easy pickings for Herc.

I would probably avoid bringing Kiyo since she'll die pretty quickly and wont do much damage with her AoE NP. Class advantage really helps in this fight because you have to last for a while due to Gawain's defense buff. Having a team of archers slowly whittle down Gawain is probably the easiest way to clear this fight.

All of the 3 star archers are pretty good and worth leveling. Robin has his NP nuke, Euryale with the male stalling and David with his team wide evade and attack buff.

Looking back, David might have been a better alternative to Robin despite the lower DPS. Makes me wish we could replay boss fights to try out different setups.

For support servant you should aim for Following servant instead of Tesla:-

Orion - Anti Male with good defense buff and evade skill. His NP will drain charge
Summer Arturius - Best single target Archer so far. Able to cast her NP back to back. Lack of evade skill but your David should be able to help with that.
Kuro von Eizbern - A weaker version on Summer Artoria but have evade skill.

Although Berserker has high damage but they quite fragile due to class affinity. You should abandon those berserker(use them for farming) and raise counter class/normal class servant since they survive better in boss fight than berserker.

Recommended F2P Servant for this fight as follows:-

Euryale - anti male servant with some stall ability
Robin - his NP is some of the best nuke around. Especially when enemy is poisoned and he got art buff on him.
Billy the kid - The gunner got some of the highest crit damage as long as you can provide him with crit star. Has evade and NP battery
David - he is reliable support servant. His skill is personal def up + heal, 1 hit evade + minor heal and charisma. His NP has skill seal effects.
Kouhai-chan - She is the most reliable F2P servant around. Def up, target able invincible and taunt with NP raising your def can let you damage dealer survive longer

Key damage dealers here are Euryale and Orion, they are anti male units, and the best option to this fight. David will also help thanks to his 1 turn evade.
You can try to charm lock him with double euryale and stheno if you happen to have your own stheno.
If all this fails just borrow a Cu Alter from your support list, most of these losers knights can be soloed with either Cu Alter or Heracles.

You really want a Euryale on this one, since she can drain NP and charm him into oblivion. Child Gil/ David also will help, gil with his charm and David with the team evade. For a support, see if someone has support Euryale, if not, Orion, Stheno, Or Marie Antionette would be the best choices for tackling this with their anti-Male kits.

Hey I think everybody else already gave you good advice when it comes to team recommendations, so I'd just like to reiterate the most important thing: do NOT waste SQ in a story boss battle! SQ are very hard to get, especially as you get further into the game. It i much better to wait and try again with better support. I won with an NP5 Kuro support. Also have a friend with an NP5 maxed skills Archuria so I got lucky there.

Wait for 3 command spells.
Borrow a kuro NP5 support or Archer artoria with at least NP2 or 3.
Enter battle and immediately NP all the way with command spells. Recommend basic mystic code so you can give evade and attack boost to her.
Use carmila to drop gawain's defence as well before NP. Buff them with Caster Gil arts buff. If you can get Tesla's NP off as well that would be great.
Beowulf last in line with an evade or invincible CE if you have it to help you across the finish line (if needed).