1 hit KO moves are not bad, but it only has a 30% chance of hitting.

There is a way to get a pokemon that has a 1 hit KO and a move so that the move doesn't miss.

I have a Smeargle, and what I did is face fisherman Ralph at route 32, the one who has a Qwilfish and Seaking, and I had my smeargle use sketch on Seaking. It sketched agility. So I knock out his Seaking with a different pokemon. After that you face some pokemon in the wild, or find a trainer to rebattle, or if you have enough rare candies, you level up smeargle 10 levels. You face the fisherman again, and keep using agility, until the fisherman uses Horn Drill with Seaking. Hopefully you have a focus sash (I think its focus sash, its one of the items that name begins with focus, that way he doesnt completely knock you out) The turn after the fisherman used horn drill use sketch, and then either knock out his pokmeon with Smeargle or a different one.

Then after that you have to level up Smeargle 10 levels again. I would say to just go to the move deleter and get rid of agility and get sketch, but it comes in handy when trying to do this. Then on the day that you can rematch Jasmine (Wednesday Aftrenoon) rematch her. Eventually she will pull out a Magnezone and again keep using agility till her Magnezone uses lock on which will make so the next move wont miss. So after she uses that use sketch. Then knock her Pokemon out. I did this and my Smeargle is at a level 41 and knows agility,horn drill,lock on, and sketch. But this is not a cheat, and it may take a little while to do but I think it is worth it.

If you don't have Jasmines number you can find her everyday between 1 and 2 at the Olivine Dining Room, talk to her and get the number. Then on Wednesday afternoons call her for a rematch and go to the Fighting Dojo in Saffaron. Also if you dont have a Smeargle, you can get one in the ruins of alph outside in HG and SS or in Diamond,Platinum,or Pearl at route 212 using the Poke Radar.