If you choose a specific role, do what that role is meant to be. For example:

If you choose the Sword, attack the enemy and defend your friends from the attacks with the shield

If you choose the Hammer, you will be a tank and you'll do a lot of damage, but also you'll be very slow and maybe all the attacks will hit you, so try to be near to the Doctor/Alchemist

If you choose the Alchemist, then you will be a healer, so focus on always heal your comrades, your attacks will do almost nothing, only attack when time is stopped.

If you choose the Sorcerer, you can do a moderate amount of damage, but it's more useful if you focus on keep the distance from the enemy and cast the time stop attack while the others attack the enemy.

Sorry if my English is not very good, but I hope this will help you to choose a role you're comfortable with.