Need help for leveling my leader

So I don't know if it was just me playing wrong or something, but I used my leader for basically every wonder, marauder and strategic resource fight, but after just winning a game via Magic Victory on turn 148, having tons of armies to defend while waiting for the 15 turns to pass, my leader on the turn of victory was only level 10.

I was playing a Large map with just 4 empires (myself and 3 NPC's), common ancient wonders, subdued umbral abyss, small underground, rare free cities and low World Threat. As a result there were a lot of fights everywhere but were easy to take care of, I even went to war, almost wiped out that empire, went to peace with them, then later destroyed them entirely and still only level 10 leader.

I watch playthroughs of this game and they normally have no problem getting to the max level with their leader and then some, so what am I doing wrong? I did let enemies run the few times it asked, but that rarely happened so much it was inconsequential. Is there an option somewhere to increase EXP gains or something?