The best way to do this (now that poison has been nerfed) is by worshipping a god which has healing, either Glowing Guardian or Dracul.
For my last run I picked human using trisword and had glowing guardian, this worked pretty well. You'll have to base your levelling strategy around whatever glyphs you find. Make sure don't convert fireball, as one of the bosses is immune to physical.
With glowing guardian you want to worship him right from the start, taking humility as soon as you can. Level up only on higher level monsters for extra piety and get to enlightenment, this won't be too hard as you'll be converting all potions which gives you piety, you'll also get piety for converting glyphs and items, and you can use sanctify as well. Take absolution boon once you get to level 7 or so, the massive amount of hitpoints he gives will help him heal you better when you take on the bosses as GG's heal is % based.
Take on the magic immune boss first, try to tackle him about level 7-8 when you're 1 kill away from levelling up, that way you can use the levelup to heal mid fight. You should be able to beat him with only 1-2 uses of protection boon, save 6 mana as the physical immune boss will be reduced to 1hp once you kill the first boss and you'll need 1 fireball to kill him.
The final boss is actually easier than the first one, you should have enough piety for at least 1 more cast of protection, and hopefully you'll have enough tiles to heal/mana to full before fighting him. You might need to convert your items and desecrate altars to scrounge together enough piety for another heal, I was able to do it without needing to do any desecrating, but I did need to convert the tower shield I bought and my fireball glyph.
If you find yourself with Dracul instead of GG, try to worship another god, like Taurog or Binlor first, this will allow you to build up to 100 piety very easily (Dracul is quite hard to build piety without sacrificing your health) then convert to Dracul once you have 100 piety, make sure you kill undeads before converting. Dracul gets piety from converting health potions and heal/sidestep glyphs so save those until you convert too. I usually take Dracul's shield ability (20% resists) as soon as I convert and then save all the rest of his piety for healing. The priest has quite a bit of spare health which you can sacrifice for piety with Dracul if you're desperate for heals at the end.
I hope this helps!