So, there is this guy who has been screwing up my life for years ( ever since elementary school). Last year I met this girl, I've liked her since then. Two months ago I told her I liked her after she broke up with her "dream" boyfriend. We've been taking it slow getting to know each other better waiting until we were both over our exes. Then all of a sudden this douchebag begins to cockblock me and I know he likes her. And it's not even small ones. They are some of the most epic cockblocks ever. He retardedly told her he liked her today, and now her head is even more mixed up. The guy's a ing . He deathglares me all the time because I stopped his cockblocking by being more aggressive. I'm sloser to her and know her more what do I do?

Epic cockblocking? Very helpful description. You use the word 3 times here and I have no clue what he actually is doing (but I'm not sure it matters). He sounds immature but you are a party to it, with all this aggression back and forth, like two rutting elk. And if she is all mixed up now, that's her own doing. She's not a quivering doe caught in the headlights. For all we know, she enjoys being fought over. Many women do.
You don't say how old you are, but all 3 of you sound like teens.
What do you do? Steer clear of him and let her make her own decisions. Stop wasting energy and testosterone on this 'enemy' who wouldn't be one if you didn't act like one too.

So what you are about 14 ? Life gets better when you realize that no body will ever steal a girlfriend if she is a real girlfriend, nothing he can say will make her leave you