Masamu is by far your poorest card. This week, with plenty of SOA predicted,
Nahomi should be there instead. You might also consider
Bruce, who could replace
Archibald or even
Trish depending on your play style.
I'd personally prefer to replace two 3* with
Dean and
Gatline (although I like
Burger when SOA is not so rampant).
La Junta is all about picking your battles, knowing when to pill and when to let the opponent hit you. The key is to win the correct two rounds so that you end up with higher life than your opponent (or KO him), because this deck isn't too likely to win three rounds. In many ways, it's a tricky deck to play. When you're learning the subtleties of
La Junta, a 50% success rate is about right, so no, you're not a bad player.
Alternatively, you could take carson's suggestion and throw in
Ambre (for
Emeth) and play the deck like a battering ram.