sometimes a Torx bit will still get a bite when a hex key won't anymore. Depends on how bad and how deep it's stripped. Like crxess says, if you feel it's getting a little bite, tap it in snug (like with a li'l hammer) before you take yer shot. Might just get one chance with it.
It really is worth getting a good driver for your grubs. MIP Thorp 1.3mm might be the one. (this size will work on many grubs/atties)
On the Torx, probably a T-6 size. You can grind the tip of it a bit if you feel you need it a bit "bigger"/snugger.