I ended up killing him the same way, with a manyullyn rapier rather than tart, and fire resist potion and naked (he can't hurt you anyway). And I presume one could use something like iron for the rapier, as long as you deal 4 damage per hit to any other creature, you will to him.
As far as the making it harder than it is, I wanted to find a way to kill him using ars magica, in the manner he was intended to be fought. I had to settle for cross mod methods because I could not win the fight. I had a solution worked out to mark/projectile/recall and move him out of his storm, but the thing is even if you respond instantly he will be fully or nearly fully healed before your projectile can reach him. I'm sure I could have tried other ways, such as pre-marking a location and using self/aoe/recall to more quickly move he and I both, but I got tired of experimenting.
And for red orbs, nature guardian is easier! Learn summon dryad, dig a 3x3 pit 2 blocks deep, jump in, summon and kill the dryads till he shows, jump out. Stay just out of melee range and he won't teleport. then you can kill him with projectile fire spells, strafing left/right when he throws the scythe to avoid it. Probably takes the same amount of time as the fire guardian, but you don't need to pay water essence, and you get some magic experience for both the dryad summoning and fireballs!
