[Vilk]

Beholders attack with magic and evil beholders has their special talent that can be disturbing. That's probably not a tactic available to you but target level 3 plus stoneskin at first round with Archimage shield is a good tactic to use on Archimage that are magic protected from base. You could even try this with stoneskin at second round if you cannot cast two spells.

If you face only normal beholders then target level 2 is enough.

If you already has lena ball a good move is also use it in this way: has a unit that wait end of turn and take care to let open some long straight path to enemy unit. Then before moving your last unit throw the ball where it is the best to damage highly one Beholder stack. I like combine this, when possible, with the tactic mentioned above against beholders. The lena ball does 100% more damages for each square run until enemy unit is hit.

EDIT: A secondary option against evil beholder is to use mass haste to have all your units moving before a large stack of them so this pack cannot use its special attack against one of your units. You probably didn't organized you character to be able to do all of that but that's definitely possible to have at this point of the game and quite sooner, except the lena ball.