You fly your Thighmaster into say... Medium range to drop the difficulty down by making the buildings less effective cover.
The Thighmaster isn't a real heavy craft so it uses the same stats at the generic airspeeder in the Core book, just with an added autoblaster.
At this range our Mercs (equipped with blaster rifles) can now shoot back. As their difficulty pools looks like...
2P for range
upgrade for urban area
1B for dusk
So total of 1P 1R 1Blk.
We'll assume they have a skill/ability of 3:2 as well... Since you are vehicle scale and their blasters are personal scale, they need to score 10 points of damage for every 1 point of HT they want to knock off. With their weapons base damage of 9, that means they'll be able to knock off a minimum of one hull with each hit.
Some quick rolling shows these guys will be hitting roughly 50% of the time, so there's a good chance they'll be doing some damage before being wiped out. If the dice roll in their favor they might even be able to score some crits and maybe even down the Thighmaster.
Now, if we beef up the Thighmaster to use the T-16 stats... with its armor of 1, it will be largely invulnerable to anything short of a heavy repeating blaster, missile tube, or proper AAA. (I don't have my AoR book with me, but I'm assuming the anti-armor grenade+launcher couldn't do it, but I might be wrong, and we'll probably see more heavy weapons as AoR supplements come out, but that's no help now..)
But again, we're back to the issue of encounter design. If the player likes to cruise around in a T-16 and the GM isn't planning for that and selecting opponents and locations that both highlight and challenge that concept, well it's not the fault of the game mechanics.
I don't think this is your problem, it's the player's problem. If the player is actively seeking to ruin another player's game they aren't welcome at the table. If the droid player thinks he's being "in character", then he needs to figure out his own reason for not turning the Wookiee in. Why should you do the work? If you impose something like a restraining bolt, it becomes an antagonistic game, and he'll just use a triumph in the next combat to knock it off or something.