Say I team attack with hawk, dove, and roy on Julia Carpenter (7/7). Before resolution of the attack, I play Teen Titans Go! and let it resolve. I exhaust dove and hawk to make Roy 8/3, then use Roy's stun ability on Julia.

Before the stun ability resolves, my opponent plays Nasty Surprise on Julia, making her 12/7 (so that her power is greater than Roy's).

Can I play another Teen Titans Go! and exhaust Roy, dove and hawk to make Roy 14/3 so that Julia gets stunned and my three characters ready?

Yes, that works fine. Is there a particular reason you thought it might not?

Because I've only been playing for about a month, and I played a more experienced player last night and a very similar thing happened (except I was attacking Killer Croc and the Nasty Surprise was a power-up, I think). From my background on MTG, I figured that my play would work. However, both my opponent and another experienced player disagreed.

Sounds like they were trying to take advantage of your relative inexperience. You would chain the 2nd TTG to the Nasty (or Power-Up) and then all things would resolve as normal.

If done right, there'd be no problem with this. I'll assume you allowed each of the initial ATK pumps to resolve before using his ability ("Activate, KO a resource you control -> Stun target character with ATK less than Roy Harper’s ATK."). His ATK was above hers initially, so the power's use is legal. While it sits on the chain, her ATK is to be raised by Nasty Surprise (to 12). Two options that are available are doing what you did: raising Roy's attack to 14 or readying and re-using his power so it will resolve before the Nasty Surprise. The former works since the power only has to be able to work at the point it's placed on the chain and the point it attempts to resolve; NS does not automatically negate the effect. If you couldn't raise his ATK sufficiently, placing a new instance of his effect on the chain will allow the shot to sneak in before the NS gets to make her ATK too high. It'd take another ATK pump on Ms. Carpenter to stop that shot.

So, in short, the only time Roy's ATK needs to exceed Julia's is when he's activated for the ability, and when the ability begins to resolve. Just raising it in the interrum does not by itself stop the effect from working.

I think he is tryin to find out if they ready or not.

Well, if that's the question...

If the defender is removed from the attack, attackers in the attack upon its conclusion will ready. So, it'd all be gravy as the question was asked.

Even though you haven't been playing long, it seems like you've got your bearings about you pretty well. :)

Well, I've been playing MTG for years and I'm something of a rules guru in that game. VS is very similar.

Anyway, many thanks to all those who replied. Any chance of showing me the relevant rules for this, or at least telling me where I can get a copy of the comprehensive rules?