Gear combat can be stupid at times, especially since it's not very well explained. Tips:
1. Near the beginning, when you first get to the desert town, the plot pushes you into the desert. Make sure you learn at least one deathblow before then.
2. The game acts like any combo earns you stuff through repetition, but only the ones that end with X do. And when you get to some of the really late-game ones, not every combo that ends in X, either.
3. Buy the most powerful Gear upgrades you can, every time you come to a store. Grinding does basically nothing for Gear combat except increasing your MP or whatever, which for most characters, doesn't matter much.
4. The game doesn't explain this anywhere I could find, but you can activate Hyper Mode by getting to level 3 and then defending. The chance that it activates is given in the HUD. Hyper Mode lets you do really powerful attacks every turn for a few turns.
5. You're not really given any good options for recovering Gear HP, so you mostly just want to go wide-open offensively. But save your energy in dungeons because you're not really given any good options for recovering that either.
All I can think of ATM.
You know, if we didn't have the Gears in Gears, I likely would have enjoyed it a LOT more. I got to a Gear fight I could not fucking win, and just gave up. ---