It can be used as a TR sweeper, but it's not a good one.
It is true Shuckle is really slow, but two main factors limit it's usefulness. The first of these is that Gyro Ball have a maximum base power of 150. Now that is quite hefty, and because Shuckle is so slow it will often hit with that full force. The second issue however, is that Shuckle have terrible Atk, as terrible as its Speed. A strong move is useless if the Pokemon that uses it is weak. Combine that with no STAB boost, poor coverage, the need for setup and being crippled with little to no effort Shuckle can be a TR sweeper, but it really shouldn't.
Bonus round! Things Shuckle can't kill!
252+ Atk Choice Band Shuckle Gyro Ball (150 BP) vs. 0 HP / 0 Def Pikachu: 90-106 (42.6 - 50.2%) -- 1.2% chance to 2HKO
252+ Atk Choice Band Shuckle Gyro Ball (150 BP) vs. 0 HP / 0 Def Aegislash-Shield: 31-37 (11.8 - 14.1%) -- possible 8HKO
252+ Atk Choice Band Shuckle Gyro Ball (150 BP) vs. 252 HP / 172 Def Clefable: 188-222 (47.7 - 56.3%) -- 29.7% chance to 2HKO after Leftovers recovery
252+ Atk Choice Band Shuckle Gyro Ball (150 BP) vs. 252 HP / 252+ Def Sableye: 78-92 (25.6 - 30.2%) -- 0.5% chance to 4HKO after Leftovers recovery
252+ Atk Choice Band Shuckle Gyro Ball (150 BP) vs. 236 HP / 0 Def Blastoise: 45-53 (12.5 - 14.8%) -- possible 7HKO
Source: http://pokemonshowdown.com/damagecalc/
