the answer is steel shade armet and always has been
a merc with guardian angel + steel shade can stand afk in an unlimited number of hydras and is just getting healed by them. Even without g-angel it wipes out most of the dps.
they have different hydra skill levels but gelebs is the highest at 135-164 per shot, reduced to 17% in pvp and doesn't scale with playercount
That 149.5 avg damage is reduced to 37 after 75% fire resist, and 15 after 90% fire resist, and the +11 fire absorb on steel shade negates/heals 22 fire damage after resists- so without g-angel or vex rune it blocks about 60% of the damage, but with g-angel it makes hydras heal your merc for about 7 hp each on average
Surprised this hasn’t been brought up yet, probably dropped/sold that item 1,000 times without remembering its properties. Sounds very promising thanks