The dungeon is balanced around getting all the help Einar can give, including the noxious brew from the northwest room on the second floor if you have at least 7 AGI, that skips the annoying kobolds, so you can keep resources and the HP and MP bonuses from the bonfire kit (again, thanks to Einar for having infinite kits) to have an edge for the postulant and the sinner phases.
I am assuming you are playing 0.37, so no new skills, but that's fine, my guide was written before those, and i developed a strategy around stalling with defend+nightstar sachet and exploiting their holy weakness, here:
postulant boss fight: the “true” boss’ first phase has 800 HP and both good defense and attack power on all 3 moves, especially his defense-piercing one, with a bit of a weakness for the holy element (so holy fist) that players with low ATK can exploit to some extent; he is another reason i recommended the nightstar sachet, as it still somewhat possible to stall and heal, get a holy fist in every now and then, and heal to full HP and MP before dealing the final hit, keeping as many potions and soup as possible for the second phase, otherwise, it’s a straightforward battle where you attack and heal alternatively, or either defend to chance some stalling/normal attack if a skill is on cooldown and your HP and MP are high
sinner boss fight: he effectively is the postulant, but with higher stats overall and 1000 HP, which means THIS is where you gotta go all in with your main way to deal damage and use your bind up and all the potions and soup you managed to save up! While it is true that 2 out of his 3 moves are still guardable, his piercing one is so strong that stalling with the sachet is a bit too risky, so just deal and heal damage fast! (although one can always chance a guard when he needs to stall urgently)
If you don't have the sachet (you buy it from Chet and upgrade it by praying at the statue), i guess you'll have to rely on your items more and luck when defending to stall for holy fist or bind up, wolf stance might allow for a few more lucky dodges if you are not attacking physically.
In 0.38, one skill inflicts armor break and can be good for strong physical builds (albeit risky), while another lets you stall to recharge MP, but it leaves you open for a turn and has a 6 turns cooldown, works better with high MP builds.
If all else fails, save scumming is always an option: save in a quick save slot, play your turn, if you don't dodge, quick load and try again
The postulant and sinner had higher DEF and MDEF in early builds, while the kobold could spam his net every turn, Caro really goes hardcore sometimes lol.