TROLLS are heavy infantry unit, at least the games says so.
and olog hais etc climb ladders etc etc
for me the best vs them was gondor infantry or cav. charging (never could back-stab them, bcs they get between my infantry >_>), even better than fountain guards, what i did on my campaign was just forget about trolls, let they deal with 2-3 groups of infantry and attack mordor orcs with cav, they will root as kids.
altho i never faced a full stack of trolls u_U, at end-game i auto-resolved with 3stacks vs 1
also... shield walls vs trolls? :S, they throw your units to air and break every formation, why shield wall? :/