It seems that as of the 1.9 combat changes (specifically the change to how Strength is applied), that the Gaia Guardian is insanely bullet spongey.

For reference purposes, I made some minor changes in 1.9:

• If the GG teleports on a vector that intersects with a player's BB, it deals damage to them
• Magic missiles deal magic (durh) damage instead of physical
• GG can now be injured by projectiles, but still has the same hard-cap on damage. Did this because all my fights ended just being a very annoying (and blind and damaged) game of tag, which wasn't fun.

and yeah Strength got nerfed back to a reasonable linear buff instead of the ridiculous multiplicative insanity it was from 1.6-1.7 (it was damage *= (1.3 * level), which is absolutely insane when stacked with enchants. now it's damage += (1.5 * level))