Its not an AV killer now, but it can be. As Werner pointed out, defense is key. Focus on sets that offer typed defense (smashing, lethal, energy, negative, fire and cold) Kinetic Combat, Aegis, Eradication are a few that come to mind. Secondary focus should be on HP, Regen and Endurance.
Willpower doesn't need soft capped defense to kill AV's. My Fire / WP was built to be a solid general purpose scrapper. He has around 30 - 32 defense to all types and I am 6 for 6 on my AV fights with no temps or inspirations. I know that perfect streak won't last, but it does show that he is quite capable.
I have a AV killer build on mids that gets his defense to around 40 % but I'll probably never try it live cause I'm pretty happy with him as my "Jack of all trades, master of none" type scrapper.