Hmmm... no multiplayer. Looks like you get to do AI matches, no problem, here are a few tips. (Note that coins usually come at a fixed rate, there are very few ways to make it faster other than to just play.)
All-Star - Try to do gnome bomb with him. AI tend to bunch up near bomb points and this is a perfect chance to use imp punt for some good vanquishes. Also, use sprint tackle, you get a 100 exp bonus (I forget if that is at x2 or not) for each enemy you hit over 1. (So 3 hits is a 200 point bonus, plus any for vanquish.)
Scientist - Start a gnome bomb match with all imps on your team, enemy can be whatever you'd like. Your only job is to revive imps as they fall for a 100 point bonus. (again, not sure if that is with x2 of not) In 15 minutes it is easy to get over 100 revives which adds up to some very nice exp. (Plus playtime coins) Of course, you can vanquish enemies too for more exp. I just recommend you let the AI do the bomb stuff. (Games will end very fast if you handle the bomb.)
Engineer - Play TT and keep the teleporter up. You'll get points each time a teammate uses it. You can also take advantage of the engineer's splash damage to wipe out several enemies on the point at once. (All of which give a bonus for capture vanquish.) For this I suggest you set the enemy to sunflower. (Rose works too, but just pick something easy to take the point from.)
Foot Soldier - When I did AI matches I did Vanquished confirmed with him. The bonus for collecting the orb on top of the vanquish helps exp move along. Sadly, I don't have a better tip than this for him.
Super Brainz - Like All-star he excels in groups of enemies. This is true for the super ball, tornado and kick. So again, set up a gnome bomb game and wipe out the clusters of enemies as they defend or attack a point.
Imp and Pirate - Sadly, I don't really have tips for these guys. I know of a trick for Imp but it requires another player. As for Pirate it took me weeks to force myself to grind his levels because it was so slow and I hated it so much. I guess vanquished confirmed will work for the same reason it does for soldier. These two just don't excel as much as the soldier so exp will probably be a bit slower.
Hope that helps, sorry about imp and pirate maybe someone else has better tricks.
I forgot to add, make sure you keep your exp multiplier maxed out by doing quests for the quest board. Its a long way to go to reach 5455, so moving twice as fast will make a big difference.
