I'll try my best but watching the video is easier ok so here is the thing.The boss can do dmg by itself if it's close enough to the impact of it's shot.So if you block the cannon with blocks u can make it do damage to itself.It does around 10 damage and whenever he firest if he is near a block he will do 10 damage to himself.Make sure you protect yourself from his minions.
The easy way I killed it was to take it to an avian airship and have it attack the avians. Then all the avians shot him down in like 20 seconds.
