As willmatter pointed out, this is not a good English. However you explained this is a dialogue. In spoken language grammer is often neglected, so this is OK as is.
If you want me translate willmatter's suggestion, I will end up in same expression (answer).
Nonetheless, this is formal expression, so you can say more coloquially like ...
僕は君を一生幸せにしてあげる。(assuming speaker is a male, which is most of the cases.)