The milestone only mentions I should use the technique in combat to increase its TL (long maxed) and I can't even find Sucker Punch in the records. And the technique tab doesn't have any hints on how to learn new ones.
Technically there is a second entry there, the Core, but that's not a combat technique. I guess maybe there's some in the elemental villages (only starting on those now), but then that's a long way after the first.