Toxic is an understated move among amateurs, and a deadly weapon in the hands of a pro. Causing gradually-worsening pain from serious poisoning, Toxic is usually used in conjunction with moves that raise the attack's evasion. Defenders may succumb to their wounds before ever getting a shot off. This is no less true in Pokemon Black and White 2 than it was in any of the previous games... though first you have to find Toxic.
Snagging Toxic is not as straightforward as it may sound at first. It's located on the first floor of the Seaside Cave, just north of Undella Bay (though you'll have to loop around to Route 21 before you can get in). The first floor is not completely accessible when you first enter, and you'll have to descend into the cave to reach the rest. Make sure you have Strength on hand to get through the basement.
Once you've made your way through the underbelly of the cave and returned to the first floor, check the northern section of the cave, past the exit in the eastern wall. You'll find an item tucked away in a corner. This is TM06. Grab it and leave.