Ranger: [wiki]Zelnehlun Fastfoot[/wiki] easy farm with N/A; for build check out the thread here in farming on Rilohn Refuge Spider Farm (it doesnt say hard mode, but its in there, I use a variation not in the thread, 16 blood, 10 shadow arts, 8+1 or +3 soul reaping: spoil victor, signet of sorrows, awaken the blood, vampiric swarm, dash, dark escape, shadow of haste, and shadow refuge. Theres a video for how to do the run in the thread.) For the run use high health armor, then swap to 55 gear when you pull (or after, but it's safe). funny thing is best bow is his green for pulling.
Warrior: Personally I like the Hulking Stone Elemental boss: [wiki]Cobble Poundstone[/wiki] but he doesn't spawn every time. Alternately I go for the minotaurs outside of Ice Tooth Cave, there's a level 27 boss there, sometimes. (any 55 damage build for these guys, or pick another warrior that always spawns)
Paragon: Newby Island, Nightfall any 55 build, Plains of Jarin, can't remember his name, he's got a few popup friends though. Alternatively [wiki]Jishol Darksong[/wiki] doesn't much like [wiki]Spoil Victor[/wiki]. (Mo/N or N/Mo 55 with spoil victor) but bring lots of regen as you'll be on fire a lot! I tend to run with both healing breeze and blood renewal.
Necro: My opinion is the easiest target is [wiki]Craw Stonereap[/wiki] go 55 with spoil victor, change to high health armor after clearing the way to him, and SV him around the corner

Dervish: probably easiest to just kill Bubahl Icehands or deatheater by clearing with heroes and henches and sending away for boss. (Mo/N or N/Mo 55 build with your high health armor - I tend to run vamp gaze or blood of the aggressor in addition for self heal/extra damage)
Edit: put in build details.