Input the following seed in the Community Challenges section.
This set up gives every friendly unit Damage Shield 2 and Multistrike 1, including the champion. You will want to start with the Burn Bright champion.
Enable the trial on the first fight then take Molten Encasement from the first pick and Shattered Shell on the third. These are all you really need for this build to work and since you're on covenant 25 you need to thin down your deck I advise against picking up too many additional cards. I also recommend hitting the Vortex whenever possible to get rid of the healing cards and additional conductors.
Now that you have these cards here is the idea of this build. You will put Molten Encasement in front of your champion and Shattered Shell so when it dies it adds stealth to them and they take no damage.
Take the right path after the first room to apply quick and Heartstone to your Shattered Shell. This is so that Shattered Shell will be able to survive hitting enemies with spikes.
Take the Purifying Cleanse after the second fight to ensure your champion stays alive then take the left path to add double stack to your Engulfed in Smoke and reduce its cost by 1 ember.
The last card you should take is Intent on Death from the first boss fight with Talos. This allows you to trigger the extinguish ability on your Molten Encasement without killing it, letting it trigger a second time on death
I recommend taking the card draw upgrades since you can get 3 extra ember by summoning two 0 cost dregs. Make sure to place your units on the first floor unless it only has 4 capacity, then move up to the second.
Enable the trial on the level 4 fight to get the Wax Snuffer. This triggers all extinguish abilities an additional time. After this fight take the left path and upgrade your Molten Encasement with Wickstone which gives it Burnout 1, ensuring it will die the round it is cast.
Finally, at the final Merchant of Steel on level 8 upgrade your Molten Encasement with Endless. This means it will die every round, triggering its extinguish ability twice and then go back to the top of your deck. Coupled with Intent on Death you shouldn't have a single monster touch you, including Seraph. I recommend summoning your left over monsters to any floor possible to remove them from rotation so you can get your spell cards faster.
This can unlock the Win at Covenant 5, 10, 15 and 25 achievements if you do not have them already as well as The Melting Remnant.