Something's buggy about this achievement- I've just had it pop while I was playing as Sonic, yet it's ignored me the last 120 times I've played as Tails.
The first thing I did with this game was play local co-op with my friend, so I picked Tails for every level as I saw there was an achievement related to playing him. We got up to Sky Fortress' boss but didn't finish the level before calling it a night and ending (15-16 Tails play there). Today I thought I'd finish the achievement so I started local co-op with a guest account being Sonic, I played the first level of the game repeatedly until my lives had gone up to 60 (from the starting 4) and noticed something was up. I exited and changed the guest account into my second tag and, seeing the threads on here, changed the level to Special Stage 1 and set up turbo controller to reselect the level for me. Fifty levels later when my lives reached 54 the achievement popped for my second tag, who had been selecting Sonic each time. I signed both my accounts out and switched my primary tag to be on the turbo controller and began again, playing as Sonic instead of Tails this time, and the achievement popped after 14 levels! The secondary account had not played as Tails before these 14 times, so I'm not sure what the trigger is exactly.