Level 100 is the max cap you can gain by getting Experience Points. All Modes, except for Practice and Tutorial, nets you Experience Points through different factors like winning, losing, health left, time left, interactibles activated, stage destruction, first hit, supers, clashes, perfects, etc. Try to finish as much of Story, Battles, and S.T.a
.R. Labs as you can since they earn you experience points. The XP Boost Items purchasable at the Archives helps drastically as well. A nice way to farm for experience points is through the Versus Mode. Basically just choose an arena where a Gadget-user can flip on an object, since it counts as interaction, it gives you free experience points. Repeat the interaction as much as possible and finish the match..
You acquire Access Cards and Armory Keys as you level up. Use these to purchase collectibles in the Archives Section.