To my memory, his upgrade paths are:
- Get 3 Stings in hand when you summon him, and a +1 ATK Incant trigger. Didn't see later ranks of this one, because I didn't pick it (which was hilarious when I was then immediately offered +10 piercing Stings as my very first relic).
- Sweep and Root! Ain't no one getting past his floor until they're dead or he is. Later ranks just up his stats. My previous record for "most roots" was beaten by an order of magnitude.
- Quick! And everything on his floor gets Quick! Later Ranks give Multistrike!
As should be quickly evident, #2 and #3 synergize stupidly well, and stacking ATK triggers also synergize with either of Sweep or Multistrike. A Quick Sweeper is everything I ever want to do in this game, and this guy just does it without praying for a good shop offer. I took the Sweep/Root upgrade to start, then later gave him Quick, then gave him Multistrike at the end. It was a very weird run, but I essentially set up shop on a floor and then just didn't let anything ever leave it. I threw some early units under the bus to tank for him until I gave him enough stacking attack (the newly buffed 3-cost rage card, a holdover rage/armour card, the +10 ATK/-2 HP card, and the new Awoken Exile base card) that nothing was every able to actually live long enough to hit me back. Seraph was very dicey (my pyre got the killing blow, but I hadn't taken any pyre damage all game prior to that since the Champion roots), since 4000 HP is a lot to burn through and he's not very good at staying alive to something that doesn't die before it can attack, but I actually had him burned down below 3k before the fight started, since my Champion was sweeping for like 200 x 2 damage by the end. The was in part assisted by a discovery I was shocked to make: bosses can be moved between floors before they become active! So I shoved him down to the bottom and Dazed him with my Trap Chute and got in some extra swings that way, which got me the win.