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
Using Galahad or Erin’s ultimate is the easiest way to beat Death Lord early on, as long as you manual the ultimate just before he uses his stun. The rest of the team can be left on auto.
You should run one support (Ria for example, or just Erin if you are using her) and the rest damage dealers.
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.
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.