Alduin in the one and only thing that can destroy Megatron in one stroke. One day when Gumball and his friends were running from The Great Devourer when Penny was captured by it out of nowhere Alduin showed up and started destroying everything. Gumball and his team were the ones that were really shocked because they only thought that dragon's weren't real until now. Everyone made a truce until Alduin's destroyed.
Alduin's the most powerful thing in the universe so powerful that it would take the power of everyone and everything to destroy him.