I've learned something simple, with the upgraded crossbow, shoot it in the face and it dies. Note that I am talking about in general, I am NOT talking about the boss.

In the actual game, they spawn in certain areas, making them less of a threat than in the following, the following, (may not be correct) they seem to spawn while your:
A: not looking.
B: after you've been there a while.

this is short, but necessary. demolishers do NOT spawn in the slums.

-demolishers will attack your buggy
-demolishers can be noticed by this: when they throw rocks (which is VERY common) you will hear a fwoosh noise, when you hear this, look for the source ASAP try to keep a barrier between you and the demolisher. (keep in mind certain things etc. wood gates, the demolisher can break through)
-dodging is required to avoid a rush. the are faster than you and will chase you until you dodge out of the way.