Yeah I had that too. I think when Megatron breaks out of the magnet's hold the feedback damages Bumblebee so the correct way to win the fight would be to:
* Power Magnet with Bumblebee
* Activate Magnet with second character
* Hit Megatron a few times with third character
* Depower Magnet
* Run from Megatron if needed and then start again...