Whatever the current stage is that you are at, there will typically be two or three other exhibition stages surrounding it. They will be labeled easy, medium or hard, all based, of course, on difficulty level, and the harder the stage, the more experience and loot you get from them. Plus, typically you will get a double EXP bonus, but you will only be able to send in one or two fighters at a time half the time.
As a general rule, though, never use any of your talismans (talismen?) in any of these exhibition battles, since these battles are effectively meaningless for anything other than loot and experience. Save your talismans for the battles that are along the main path – or possibly tough side battles.
Nevertheless, use these for fast leveling when you really need to train and/or upgrade some fighters. Also look for the timed stages, which are PvP stages. If you are about to run out of time on a timed stage, then go back to the date and time settings on your phone, and set the time backwards (not forwards) by just enough time to give you extra fighting time.
Keep an eye out for warriors who have special attacks that can damage multiple fighters at a time. Also, keep an eye out for fighters who have poison attacks that can do damage to one (or even better, all) opposing fighters at once. They are excellent to have on your team, but they are especially difficult to fight against. The only cure to the poison is to let it run its course and use healing spells when needed.
Keep on the lookout for gates that are off to the side of the main path. These aren’t entirely necessary for you to go through, but if you go through them you’ll find rare treasures as well as tough side quest battles.
The portal is a good farming location for experience points. There are new events every day that you go there, but Sundays are the best day to go for experience points, because they will be doubled the whole way around.