easy for a assassin, take [Ebon Escape] shadow step out of the circle, bear will get stuck, then use any type of ranged attack you want to kill him. spirits work, bow works, pve skills like [Ebon Vanguard Assassin Support] work
go A/R, get a bow and [Viper's Defense][Heart of Shadow][Apply Poison] and some random bow attacks. Get as close as you can to the outer side of the "arena" and shadow step. it should shadow step u outside, if it doesn't then just try again with the other shadow step skill. then just use the prep and whatever bow attacks u got and he cannot attack you. Good luck and have fun!