Level 3 Bush is stronger, but still a bush. If you wasted all your EXP fighting the level 1 bushes you won't get any for the level 3 bushes. It's a treacherous system.

I'll admit eventually it'll come to a point where even after you fight all the monsters in Deep Ground, you might not make it to even level 5. Most of the EXP you'll gain, and indeed what really ranks you in the world, is through PvP. However you'd want to do PvE to fill up empty levels as well as to gather equipment and weapons.

And if you intend to try to leech, you'd have to party up for that, and even then the EXP is divided amongst the party members based on damage ratios. So if you intend to have a level 9 warrior do 99% of the damage so the level one can get the 1%, he's only going to get the 1%, and it'll be counted towards his total NC kills. This sucks even harder if you lost level due to dying, since those kills still carry over even if you lose EXP, and will eventually force you into participating in PvP.