How do you level up in this game?

Okay so this might come off as stupid but I beat Elden Ring twice, then started playing DS3, got past Vordt pretty easily and I just reached the Curse-Rotted Greatwood and am having a lot of trouble against it. I started off as an assassin and am lvl 15 right now. I saw a post about someone being lvl 36 when reaching this boss and I'm like "how???". Since Elden Ring is open world, I'd just go explore to lvl up or get an item I might've missed that could help me then come back. I obviously can't do that in DS3. Do I have to farm dungeon mobs? I hope there's another way because this seems extremely boring to me.