What you have to do when there are multiple mobs is called 'kiting'. It means to run while keeping mobs away from you, like you are flying a kite with the mob as your kite.
So when there are multiple mobs, make sure you have plenty of space to run away. You start by shooting one of them with your ranged weapon (buy a white one from a vendor). Then when he comes close to you, hit him once with your weapon, turn around and run. Hitting a mob will reset or increase the range it takes for him to reset (lose aggro and go back to where he was pulled). The goal is to keep 1 mob with you, untill the other ones reset and go back to their spot. Then you can kill that 1 mob alone.
Another thing to worry about is getting dazed which reduces your movement speed. When an enemy hits you from behind you can get dazed, getting dazed while 3-4 mobs are chasing you can lead to certain death. Learn to '360 jump shot' as we used to call it. You jump and while mid air you turn around 180 degrees so you are facing the mob mid air, meaning you can't be dazed. Then before you land quickly turn back 180 degrees to your original direction. This way you can avoid being dazed and not lose any movement speed by turning around the normal way. Try and time your 360 jump with the mobs attack, so every time he attacks you are facing him, meaning no daze. You can even try and get an attack off on the mob mid air while you are turned around. This is a very useful skill to learn for any class who have instant cast spells they can cast while jumping.