[Twan]

One of the best ways is slay/paralyze spells as they are rarely numerous (if you have astral mages or can reach alt 8 for disintegrate)

Blood or nature can also allow you to charm them at high levels.

Or just immobilize them (with mages chain casting spells like vine arrow, earth meld, tangle vines, bonds of fire etc...) and kill them with hordes of expandable troops (troops with powerful weapons like barbarians to do more dmg than what they regenerate)

Finally, even with an E9 bless, units using n9+ bless are vulnerable against fatigue (especially niefel or fomorian thugs casting quicken self) as berserking gives an encumbrance penalty, so any spell doing stun dammage is good to use.