This boss uses a shield that reflects damage and reduces damage taken, but his damage reduction never fades and keeps stacking with each new shield it puts up.
.go xyz -433.426000 215.664000 -52.787300 48 3.933790
Haven't been able to reproduce, works fine, he occasionally casts his %dmg reduction buff and it seems to work.
However, if you tame the boss, he will respawn back in a few seconds which is really uncanny.