Dretching is a very peculiar way of fighting. It takes a while to get used to the contact damage , and even longer to effectively use the ground headshot feature-exploit. However , luck plays an important role in frantic fights - you don't have to set yourself to strike humans at each leap. Just random wallrunning and hopping from a surface to another in the immediate vicinity of humans can work just as well , since you are likely to fall on a human's head. Of course it doesn't work too well against flamers.
Dretches can hardly outrun an human , and most aliens just can't. But since dretches are silent , they can follow humans for quite a while and land a deadly hit with ease. But it is not effective to plainly run at a human who noticed you , better try to disorient him until you are very close.