It's all about enemy attack patterns, enemy movements, learning spawn positions, what enemies are in each spawn, etc. You don't have to learn it all immediately... the knowledge just naturally comes to you as you play the game but make sure to keep an eye out for it.
One thing I will say is learning animation manipulation is a really good way to get free attacks into enemies while being solo that you normally couldn't get hits on before. But that's a discussing for another time :]