Once again, with Lil' Hunter you can just melee him down with most weapons, the trick is getting close enough without getting hit. Melee range increase perk is really nice, as is the one that increases your reload speed the lower you are on health.
There's basically three ways to handle Lil' Hunter, the first is with some spray and pray weapon like the automatic shotgun in which you hold down the button until he dies (this works particularly well because Lil' Hunter's jetpack activates as a result of the player initiating an attack, therefore holding down the button doesn't count). The second is with a powerful weapon like a Bazooka or Heavy Crossbow, which usually kills him in 3 or 4 hits. The final is to just rush him with a melee weapon each time he lands, reflecting all his bullets back at him until he dies. They all have their strengths and weaknesses.
The hardest part about Lil' Hunter is all the police he spawns, but it's actually a good thing because they often drop health packs all over the place. You just have to be aware of your positioning in relation to them. If you can swing right as the portal opens you can kill them instantly.