Wow. Finding similarities is difficult. However, I guess one would be that both of them value family and relationships above all else. If you recall, when Beowulf goes back to the cave it is Grendel's mother he encounters. So there must have been a loving relationship and devestation at the loss of her son. Also, both of them fight by themselves, for themselves, without the assistance of weaponry. Both use their wit and strength to win a battle. That's as far as I can stretch it. Good luck!
