First attempt: I've also made it to 4W, but I'm in less good shape, and I'm not sure whether this is something to be addressed by rebalancing the boss, or whether it's okay for this to be a failure state and I need to go back and reoptimise.
My stats (with the Everbane Blade) are 5001 HP, 559 ATK, 639 DEF. The Architect's expected damage is 8968. If I go back to Level 6 and kill monsters that are now free (Slayers, Adders, Rock Giants), I can get the next (38k) altar upgrade, which is still not enough. I can get the pickaxe, use it to get the grappling hook, and use that to get the hand bomb (from Level 6 2S), which allows me to win the level, but it's not very satisfying
Second attempt: I replayed Level 7, this time taking only DEF at the altar (which I used, iirc, three times). Final stats (with Everbane Blade): 10659 HP, 483 ATK, 695 DEF, 6/5/1 keys. Architect does 9568 damage. So a DEF route (at least, taking DEF in Level 7) is greatly superior, since the boss's damage is about the same in both routes, but the DEF route makes it easier to get through the level using less HP.
So the boss might be okay, with the proviso that the current stats make it pretty much mandatory to use the Everbane Blade, unless you are a really good optimiser.
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50th Skywatcher