Onslaught is a source skills, of course it's strong. And by the way it costs 2 AP not 3 AP -- there are no 3 AP cost skills.
I won't debate you on "amazing defensive stats" since it is subjective -- I was just trying to illustrate having a shield could be the difference of life and death (or being crowd controlled).
Says 3 AP for me in the game. And oneshotting everything isn't strong, it's broken. More so with dw/2h when you do 2x or more damage compared to 1h.
There's no way in hell a mediocre physical armour bonus is worth halving your damage.