The most useful monster.

That's impossible :) In the first turn, the angels won't reach, and the beholders attack in groups. Besides, they aren't alone; sometimes they come with Minotaurs.... So, if the beholders fail to inflict damage on the Crusaders, and the Minotaurs attack them, then...

If there are angels, it means the tactics are already at a higher level; they will most likely reach. You can also use Sanctuary to get them close. Although, the most important thing is the Amulet of Chaos; then, the beholders won't be a problem for the Crusaders at all :)
P.S. I hate Minotaurs; when you fight against them, they can reflect four attacks in a row, but when they are