Energy is what allows you to battle in the single player modes and in the ICL, and each time you battle, you use up a little bit of it. The tougher the battle, or the more major (such as a hero battle or an ICL battle), the more energy you use. Once you run out of energy, you can use an energy drink, and if you run out of energy drinks, you can find them as a prize for battling in the PvP mode.
You also have PvP battle points, and once you run out of that, you can restore it using a Battle Drink. Ironically, the main way to earn battle drinks is to find them as a reward in the single player mode. You can both types of drinks here, but the battle drinks pop up far more frequently.
This game tends to have a very spotty record of connecting. Ironically, the record is actually worse when you are on WiFi as opposed to being on 3G or LTE, so if it has problems, shut the WiFi off. Of course, if it has more problems without WiFi, then turn it back on.
Your team rank will increase as you increase your team’s power using sparks. Every time that you pass a specific milestone, your team power will rank up by one. Your team can rank all the way up to XV (fifteen) and you’ll end up unlocking the equivalent ranks, unless you just end up winning via the PvP mode rewards (which, of course, are quite plentiful).
In general, the recommended power level to take on an opponent is a good guide to go by. It’s not always gospel, though. You can win at a lower power level with the right combination of moves, or if your main three monsters are exceedingly powerful but your fourth monster is not as much.
Each PvP tournament has its own element. For example, one week you might do the fire tournament, and the next week the rock tournament. Notice that all of the creature prizes offered up in each tournament will be of one single element.