Perks, leveling (hand in hand I know) getting the sigel of growth and pain from Trogar so you can induce growth (including muscle which adds to strength) and/or increase the damage of you next hit. They stack. If you're having trouble fighting anything at all, fighting The Bind is a decent way of gaining XP and essence. Also, Essence hoarder perk. Even as someone with a lot of back progress and achievements, the start of the game is still a bit slow in actively improving your stats. Also if you don't mind going back a version or two to build up some progress in achievements over, the slime, while a slow XP gain, doesn't scale in any manner. But yeah, choosing good perks/abilities and making sure you interact with Lukka are critical in both getting your growth rolling and just increasing your strength.