All regular Pokemon have a hard-counter they can't do anything against under normal circumstances, so in a game with a diverse pool of mons to choose from, challenging battles need to have stronger mons or battlefield advantages (IE field effects) over the player, or else they become easier and easier as you progress and have increasingly more options, they need to operate on a higher "tier" if you want it to take synergistic teambuilding and planning to beat them and not just slapping together hard-counters.

Nuzlockes need not apply, the core experience shouldn't be balanced around such a massively RNG challenge.