I teached Focus Punch to my Scrafty (instead of High Jump Kick) by a Move Tutor (at first I didn't teach it by leveling up), but when I tested it, I wasn't satisfied, so I changed it to Brick Break. What should I do till I reach Mount Lanakila? I lost my most powerful move!
The main niche for using Focus Punch is if you've paired it up with Substitute. Once you set up your sub (which works best if you're slower than the opponent, as Scrafty is likely to be), the opponent's next turn will probably be occupied with hitting the sub, which doesn't count as hitting you, and which doesn't stop you from successfully using Focus Punch for a lot of damage. The only ways they can get around that and stop you from punching them are by using a multi-hit move (like Icicle Spear) which is somehow powerful enough to break the sub with at least 1 hit left in reserve, or by using a sound move like Hyper Voice which ignores substitutes entirely, or by using the Infiltrator ability which makes all of its moves bypass subs just as sound moves normally do, or by using a ghost (which is immune to Focus Punch by typing, though in that case you can probably hit it with some dark moves instead).