Wasteland boss

Takes very little damage if any at all.....he's like immune to everything... If you do get any of his hp down he instas to full unkillable Glitch/bug maybe

Using cyclone as skill

I once played split arrow bleed and had the same experience as you.
Now im playing poison bladeflurry and are killing him in 2 seconds.

What feel and think is that he has some sort of dps check mod, and your dps is simply to low.
Todays park boss was once found on the garden map, and i had same problem there with my split arrow bleed.