Cast Target on a strong unit like the trolls or cyclops
+ Archmage's shield on targeted unit
(+ stone skin if you're a mage with higher magic)
= recipe for success against archers
Perhaps the best thing about target is that it forces all those pesky ranged units to use their (often puny) normal attack and forego their special attacks, which are often the most damaging.
Against shaman & alchemists I recommend magic shackles, though leadership plays a role here.