Hi you bros, I'm Niko and I have a simple tips on killing Gate-Crasher. In a normal way, we invite friends to help because we can't beat the GC, what if we don't have any friends online?
GC will be level up if you spend 3 attempts on it and it got killed (kill on your own without any help), it won't level up if you have a friend who fights along with you.
Once you are strong enough to kill GC on your own. My tip is to create a noob account (level 30+) and invite it to your Home to spend 1 attemp on, after that kill it and the level will stay the same.
Hope it help!