In the Community Challenges section input the following seed.
This set up gives every friendly Damage Shield 2 and Multistrike 1, including the champion. You will want to start with the Glutton champion.
The overall idea is to place the champion on the top floor and feed him as many morsels as possible. You will want to pick up 1 Morselmaster (creates a copy of a morsel you summon) and put him on the floor with your champion. After each boss fight take the additional capacity option so at the end you can summon 4 morsels on the top floor.
A few things will greatly improve your odds of winning and killing Seraph before his final round.
Go for the Umbra banner to get Morselmaster, you only need one.
Take Trample card when you beat Talos.
Turn on the Trial for level 4. This will give you the Fossilized Fangs artifact which doubles gorge abilities.
Take the Unstable Vortex path on level 7 to get the Shroud Spike then use the Merchant of Magic path on the final level to add +2 to the X ammount.
Finally, buy Hell's Banner from the last trinket shop which will add 3 ember when you summon your second unit (hint: morsels are 0 cast and count as units)
The plan for the final fight is to use your Shroud Spike when you have 7-10 ember. This will give your champion around 1300 damage, with trample and multistrike and should be enough to kill Serapth before his battle phase.
This can unlock the Win at Covenant 5, 10, 15 and 25 achievements as well as The Umbra and Sans Serapth if you do not have them already.