Once your Stamina is depleted, you must wait for it to recharge in order to play. To reduce this, increase Stamina when leveling up. This will allow you to play longer, earn more experience points, and further improve your characters. Complete all missions possible to use up your Stamina before you stop your play session. The next time you play, you will start with full Stamina.
Summoning
There are two attack types, a regular attack and summoning one of five characters in the battle. There is no penalty for summoning multiple times, and doing so will earn both the team and communication bonuses.
Various strategies
Transfuse and evolve your cards whenever possible to get more powerful versions. Collect the free pack of cards available daily. Use the unneeded ones as fodder to evolve or transfuse other cards. Power up your rarest characters as much as you can. The more common cards will be replaced constantly over the course of the game as you quire more of them.
Various Tips and Tricks
Your best bet when it comes to picking a weapons loadout is to take the machine gun and the shotgun. The machine gun holds plenty of ammunition (ideal for mowing down large groups of mummies). The shotgun will easily clear the path right ahead of you with one clean shot. If you want your shots to pack more power, always choose the shotgun. Alternatively, you might want to check out the crossbow - it's both effective and fast-loading. With Pair of Boots equipped, you'll run even faster, although though you'll need to maneuver much more quickly around groups of mummies to avoid getting hit. Goggles will make things much easier to see through sandstorms - and considering the first one pops up a few seconds into the first level, you'll need them. Holster lets you carry more than one weapon at a time. As the mummies begin to fill up the screen more and more, this upgrade fast becomes essential. Evasion is the best way to get around the mummies. If you run around them well beyond their range, you won't get hit - it's as simple as that.
Stamina...