Paladin, Nercomancer and both Cleric and Sage are great at soloing bosses.
Paladin - Obvious.
Nercomancer - Debuffs, Drain Health, Pet Summon and good damage.
Cleric - Buffs, Strong heals, high HP/Def, and stuns.
Sages - DoT Heals, pushback/stuns and various damage methods.
Ranger has some difficulty soloing bosses. In dungeons, they are immune to stuns. They get into close range too often and too fast for them to use their ranged skills. Field bosses though, sure, they can take those down pretty easily. Bait bosses (ones summoned by items) are difficult at the start since they have to start the match in close range. As soon as they get to level 40 and get the second level of weasel dodge it gets a lot easier.