Why?
mercs get 2x dmg received from prime evils which is plenty of return dmg for p8 boss fights especially if the merc doesnt have dr %
you can still use insight iron golem just fine with a bramble merc because of this
bramble for p8 means more consistent ce chains after the first corpse because theres more than 1 monster that is not full hp from amp+thorns
fort for p8 means merc has to get at least 2 kills before ce chain is consistent because summon dmg alone is shit
amp -> stomp target -> south walk -> ce chain -> repeat
vs
amp -> stomp target -> ce -> stomp next target -> ce chain
now you might have an argument that now a merc has a slower time to kill the first target without fort but needing to stomp a second target while waiting for a 2nd corpse to not even guarantee a ce chain sounds slower overall
ofc this doesnt matter if you are fighting souls or oblivion knights that hardly ever do melee attacks
tldr bramble makes your summons more than just a meat shield more often than relying on merc to be your main dps before ce
if p1-p3 fort is the obvious answer