Is it possible to reduce your effective damage to 0? Get knockback and enough damage reduction to take no damage yourself, then just keep smacking a monster into an IMAWAL wall to fuel Rock Heart, instead of casting PISORF. You lose the Fire Heart charge but are spending far less mana.
Also, if you can figure out a way to reduce your max mana, even by 1, you can turn the Transmuter's mana drain into a full MP restore on every conversion. Spirit Sword doesn't activate if your existing bonus is higher than you'd get from your current max MP, but you still get the mana refresh. So, e.g., do one conversion then take Wereward, which would actually kill both of these birds with one boon - and you could also desecrate/anger Taurog to get your damage down.
EDIT: Now that I actually think it through, the knockback thing wouldn't work at all with Spirit Sword, so you could only do one or the other of these. Also, the Bear Mace still hits Goo Blobs for 1 damage into a wall at 5 base damage, so I'm not sure if the knockback thing is even feasible.