How do you use Crossbones effectively?

Put him on a team with figures with commonly used team abilities, such as avengers, x-men, ultimates, etc.

The problem with building with his special power in mind is that he doesn't get it until late dial, so it wouldn't be a reliable bet to even use brainwash.

What Crossbones has going for him is his high defense through the dial and solid attack values for his points. You could pair him up with a JSA or Defender with lower defense to give it that 17. I would suggest E Dr. Strange from Fantastic Forces. He's a defender with 16 D and ES/D. If you park him in hidering he gets 20 D from range!

You could add armor piercing to make the most out of his EE and the usual wildcard abuse. He could benefit from an Ultimates TA or a Sinister TA.

Giving him good team abilities to copy really goes a long way to making him more effective. Another important thing to keep in mind is that Crossbones needs to be a second string attacker. if you run him headlong into battle I'd imagine, unless the dice go berserk, he will get cut down pretty quick.
Let him hang back a little at first and then bring him in and let him pick away at your opponents. and try your best to keep him on that first click or two.