Ice Beam is easily one of the most useful and popular moves in any Pokemon game, and given the nature of the champion in Pokemon Black and White 2, you'd be wise to track TM13 down and equip Ice Beam on one of your pokemon. But where is that elusive TM?
Like Pokemon Black and White, it's found in the final portions of the game, near the Giant Chasm. Unlike Pokemon Black and White, however, it's in the large, roundabout cave just before you enter the Chasm itself - and it's available before you take on the Elite Four for the first time, not after. You'd do well to pick it up long before you even reach Victory Road.
Getting TM13 is relatively easy once you enter the cave from the east. Look slightly west and you'll see a stretch of water running from north to south. At the north end is an item. This is TM13. Head south from here and push the first large rock you see into place with Strength. This will allow you to run up to the water, Surf to the north, and grab Ice Beam.