Often used in Halo 3, "getting beasted on" is when someone who is "beast" (pro or legit) owns the living crap out of you and pretty much embarrasses you by making you look like a n00b. This term is originally derived from the term of "getting pooped on," which is to lose a game pretty bad. The difference in getting beasted on is if this happens to you despite you having some measure of skill. Talking yourself up as pro just cause you're a level 50 and then getting raped 50-7 where one of your opponents gets 30 kills on your team is a textbook example of getting beasted on.
We thought we were good until MC Hellraiser beasted on us. We lost 12-50 and he got a 19 kill perfection on us. He screamed at us to get beasted on when the game was done. Now I wish I hadn't talked so much shit cause now I look stupid.