Most times you're being bombarded by attacking slugs. With this, you can turn the tables on them and kill them instead. Most of the tricks on this page require the attacking slug to be boosting (otherwise its not defense, its offense)
I've used this trick countless times to kill. It requires good timing and quick reflexes, and a slug boosting toward your rear-end. You have to release one ball of slime, and they usually fall into your trap. The closer the slug gets the easier the kill. When you're a medium-sized slug with around 800-1000 mass, you can judge when to release the slime by the length of your tail. When they reach your tail, they have no chance of escaping the trap, but you have to be quick or you will get rushed through.
This trick is easy, but may be difficult when there's a lot of slugs around. This requires you and the other slug coming head to head. Just before you hit each other, make a quick U-turn while boosting. This saves your life and gives you mass. There is a chance of them dieing near your head, giving you instant mass, but sometimes there will be a slug around just waiting to steal your kill. Of course, the other slug may also do a turn, so you might have just wasted some slime.
This does not kill the attacking slug and uses up some mass but saves your life. You just flee using the boost and run away from the slugs. Simple and easy!