Hi. The level-up rate for stats gets a bit longer for each level-up in the same category. So, level 2 strength will take a bit more time than level 1. Still, level 3 is the maximum, as the status screen clearly shows, and there's no point in trying to level it up higher.
Combat doesn't require grinding just timely use of your shield (slime's waterballs), running away from attacks that can't be blocked (slime's jump stomp), and attacking while the enemy is open.
If you don't know what to do next, use the quest journal from the on-map menu or Meggie's lamp from the item menu. These will tell you what to do next.