So when I play 1v1, should I just focus on spending as much as possible and always killing his relaxed army, or is there a better to way to getting more exp per game?
Cannon rush its the way to go
if you talk about warchest, then you get 100 000 exp per day once for each race for the first win, so you can play ladder with your main race and play 2 more games against easy ai with both other races, which should last longer than 3 minutes (otherwise you dont get exp), this gives you 300 000 exp, or at least 1 skin per day, if you also play coop, then 4 games = 300 000 exp, or 1 more additional skin.