This boss only has two mechanics you need to worry about; his passive and his ultimate skill.
Ultimate skill is a team-wide stun
Passive skill increases ATK by 50% on each kill
This team should work with whatever DPS units you have as long as you manual Galahad ultimate just before the boss uses his stun. The rest of the team can be left on auto.
Every character is using their own card but this will depend what you have available.
This is the most consistent strategy to clear the boss when you are close in level to him since his stun will wipe most teams.
This team only works after investment in your main damage dealers and relies on Merlin to freeze one of the stuns from the boss.
If you don’t have dupes on the DPS girls and decent gear, run the Galahad strategy instead.
Every account is different so I don’t like recommending pets but instead units you can build around.
One of the most popular pets is Esteria since the fear matches with most boss skills (20 seconds cooldown) and can be used to cheese a lot of mechanics.
Personally I use double offensive (Morgana and Athena) pet and one defensive (Kei) since that’s what I have with more dupes.
Higher stats usually means having an easier time on all fights, so use your units / pets with most dupes.