Dende with the right lp% regen or high con and reflect are kinda hard to beat and with buffs forget about it you need to have a high crit rate to kill em
Pokos - Hard to beat when they can heal with their high defense and can drain your EP to 0. And they summon pets to auto attack you while u fight
Dark Warriors are the ultimate Tankers in this game. With high upgraded mudosa gear and high con pfft its so hard to get good damage on them. They are hard to beat with their confusion skill. But they not that good for pvp but hopefully in the lvl 70 update it will change.
Shadow knight - Basically the pvp version of Dark warriors but with less defense. They have alot of good attacks like fear, life steal and eyebeam. One of the hardest classes to beat