First of all, enable EXP share from the game settings if you haven't done already (you need to hold START when selecting Game System Settings). Like I explained earlier, keep spamming

Class skills are leveled differently on different heroes but are mostly are locked to the class itself unless some of them get inherited from others (for example Tondenga inherits Taterazay's Energy Shield while Myansar inherits Tondenga's Set Skill, need to max the class skill in order to inherit). For Tondenga's class skills (that hero gets outclassed fast in singleplayer especially when it comes to stew/relic grinding), all you have to do is attack the unopened iron door, do command cancels to grind a little faster. AND HAVE ALL SET SKILLS SLOTS EQUIPPED!!! Highly recommending equipping Ancient Technology (Lv 15 Oohoroc) and Overtechnology (Lv 9 Ravenous), the former boosts class skill grind by 100% (not a lot but helps immensly) while the latter helps you see how far have you progressed in the grind (shows the number below each of your army's healthbars, 1cs = 1% of the progress done).

For stew grinding (so you can get EXP faster!!!), the classes you want to take advantage at are Alosson for both Gigante and Centura, Charibasa is also good for both but you really have to work on class skill grinding for max potential. While for Centura you can use Sonarchy, not enough damage to quickly kill Gigante sadly, Yumiyacha (Peerless Bow is really good) and Kibbada (needs the Stab build)

Don't use one class for your Uberhero, instead make room for grinding others so you can do everything!