Arcade's card states whenever he stuns a character ko that character. My question is can this be used as part of a team attack??

Yes, he can.

The same applies to everyone involved, including the defender. For example, attacking Bishop with four ranged characters gives him +12 ATK and +12 DEF.

Yes. All indiviual abilities are applied even during a team attack. The only thing I can think of that doesn't are abilities that require Breakthrough DMG.

I just realized that Arcade does not have a team affiliation. There goes that idea.

I wonder, can Marvel Team up allow you to consider no affiliates a team. Meaning can you name the x-men and non affiliates to team up?

No, it would only work for named teams. That's the drawback with unaffiliated characters - they can be very powerful, but they *never* get team attacks or reinforcement (short of effects like Burn Rubber).
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It helps when you read the card, eh? :ermm:

Well, yeah...:rolleyes:
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well there goes that idea, lol.