Beholder

Beholders are creatures with a giant central eye, and several peripheral little eyes. They are able to cast elemental shards with their small eyes, and huge elemental balls with their central eye.

Strategies
Quickly cast a cold electric arcane shield ( self) on yourself so that they deal no damage. After that either slow or freeze them to give yourself as much time as possible to deal with each one separately. Followed up by your favourite high damage spell. Preferably steamlectric beam or imbue , which will deal 4x damage while they are frozen. If you see them charging their orb of death, or want to do this safely, start with a dome shield ( AoE) and fast charge it with heal seeds ( AoE). Then use Electric Wards sword imbue while at the edge of the dome to attack them from within. If you have Vortex  or Thunder Bolt , that's just as good. You can also cast invisibility and prepare. When you are ready you can just cast it. After that, use to melt it. If you get frozen while you don't have a cold electric arcane shield, abandon everything and as quickly as possible thaw yourself. Not doing this will mean death - the beholders' elemental bolts can bring instant death even to a wizard with Staff of War or Aristo-Staff.