Spatula said:
Sounds like a BBEG encounter to me (CR-wise).
If we accept EL 12 as a good guess for the encounter (the antimagic beholder definitely addes at least +1 EL to an EL 10 base encounter and, plus traps I think you could argue EL 13 or even more given how bad it is to not use magic versus deathless frenzy) then it is either EL +4 or EL +5 for the party (which should see encounters of EL 7 or 8.
From DMG:
EL +1 to +4 (notice we are on the high end) is described as: Very difficult. On e PC might very well die. The encounter level is higher than the party level. This type of encounter may seem more dangerous than an overpowering one, because it is not immediately obvious to the players that the PCs should flee.
EL +5: Overpowering. The PCs should run. If they don't, they will almost certainly lose.
Now, if the party is completely fresh, pumped up with information and ready to rock and roll then they might take an encounter like this with sheer guts. But if they had to fight their way in, have lost spells, options and hit points then this is a death trap. If the dungeon does not allow "resting at will" and "refreshes between visits" then hitting the BBEG completely frsh may not be an option.
Now, if you allow very high point buy, gesalt characters or other such things you might be able to take on such encounters but that is because the true EL of the party is actually one or two higher than the 25 point buy core class baseline for the game.
I will also note that a Barbarian 10 could have stood in thematically and caused way less problems.