Fissure is a horrible Move, and it is not worth Lvling up a Trapinch that much for such a bad Move. Your best best is Lvling it up to LV 55 to get Earthquake, but even then, you can use the TM. You're better off going with evolve it all the way to Flygon, because I don't see how you would get a Trapinch all the way to Lv 73 anyway. Flygon would be doing more danage overall than a Trapinch with Fissure ever would, honestly.