Hello Guymelef and welcome to Imperial Hero.
Your first and second question pretty much got a common answer. You can't do anything but watch during a fight. It's just visual, it's not real time. The battle is generated and finished by the time you press the "Fight" button. All participants choose their target to attack and they keep attacking that target until it is (or they are) dead. There is a sole exception to this, the guardian's skill Taunt, which pulls the target of the enemies on the user (which makes a guardian mercenary a must in tier 3 and above groups).
Faster way to earn XP and gold can only be found by the player himself. Depends on your equipment, your skills and your team alltogether. Best idea is to find a place with 2-3 enemies where you can kill them without mercenaries. That yields a lot of XP but can be a bit costly. To earn gold, try repeating a multi-stage dungeon with a difficult last boss. These guys tend to drop nice stuff. You can then sell that stuff to the auction to other players and make some profit.
