My fellow DMs and I have come across a problem. It feels like the party has been doing a good job of dispatching our monsters with relative ease. We are not upset about this part, our party has done an awesome job of understanding their roles and maximising party potential. The problem becomes that the party hasn't been especially challenged. I attempted to solve this issue when it was my turn to DM and put together what I believe to be a tough fight. I put together various monsters that synergized well with each other and had lots of crowd control in attempts do slow the party down if they didn't read the fight correctly. The fight definitly became a challenge but I also noticed that I seemed to be frustrating some players. I realize there's going to be some form of frustration during a tough fight but it's not much fun for the players if I'm crowd controlling them to the point they can't do what their class is meant to do. For example, I immobolized and slowed one of our strikers throughout the fight cause she was a heavy hitting and mobile melee striker. I knew if I let her run around she'd make short work of some monsters. While this is great for the monster it's frustrating to her cause she doesn't get to do her thing. This problem gets multiplied when the player of course has a hard time rolling saves against conditions. Is there some way I can keep fights challenging without frustrating some of my players?
If people were wondering, I threw a beholder at them in attempts to throw out eye rays during their turns, putting slowing conditions on melee folks and hitting the "squishies" with more powerful eye blasts. I want to play the monster intelligently but this can frustrate the heck out of some players when they're consistently immobolized and getting hit hard with ongoing damages and have a bad day with saving throws.