great fun thanks for taking the time to post.
two quick comments, Great Strength - you need to spend your first action doing this to gain the STR bonus for your attack, and Wandering Monster - the defender can (max 3 times) sacrifice any Adventure Token to bring on a wandering monster (on a 4+ if I remember correctly) so your Beholder could have tried to call support rather than wait for the heroes to flip the tokens.
With regard the main fight, a 100 GP creature v's two legends and two companions (about 170 GP worth) would be tough for the denizens, remember that Legends equate to high level D&D characters. That said as OW used d6 it means that lucky rolling can have a HUGE impact on the game with low level minions serving against the odds and heroes or villains dying from slight injuries like the crossbow bolt that killed the beholder.
thanks again for posting, your dungeon looked fab