It makes a huge difference whether or not the party is expecting to fight a beholder. If you just randomly stumble upon a wandering beholder without being prepared for it, you're very likely to be screwed. But if you know it's coming, you can prepare, and it's that preparation beforehand where the real action is: Prepare enough, and it'll be easy, don't prepare enough, and you're likely to be screwed. The fight isn't the action, it's just where you find out if your preparations were enough.