It really depends on the character so it's hard to say a fast way to level. But if you are really good at PVP then multiplayer would be a lot faster since you get 200XP per take down along with other bonuses and streaks
Aside from that though it's faster to just farm the bosses in the backyard on easy mode or playing some of the longer game modes vs the AI on easy like gnome bomb or turf take over. you get less experience against the AI of course but you can usually kill them at a much faster and easier rate to make up for it. The most you can earn from them is getting lucky with ending their streak and saving an ally which gives you 150XP which is only 50 less from taking out players.