In that same dungeon, before you get the CS with Ramuh, Shiva, and Ifrit, there's a path to the right, right before the save that's blocked off by a boulder. Once you learn the smash ability, you can break that and access an area that gives insane exp for how early you can access it, and on top of that, you encounter Zapt, Bablizz, and Fritt, which are basically the baby versions that can evolve into Ramuh/Shiva/Ifrit. The exp there ranges anywhere from like 5500-10800 I believe depending on the encounter. It's a extremely good early game grind spot if you don't mind being a bit overleveled early on, while getting chance at those baby versions.
If trying to grind out the baby versions without overleveling, just keep escaping battles till you finally encounter them, they're a bit rare compared to other mirages showing. I highly suggest having a mirage with the "No Escape" ability as they can run when you finally do encounter them. Their imprism trigger is hitting them with the element they're strong against, so bring either Thunder/Ice/Fire Mirage, or just buy items from Chocolina.
Earliest you can get Smash is by capturing a Copper Gnome from Death Nebula cave, lvl it to 10, evolve it into Mithril Giant, first skill the giant learns is smash.