For casuals the PvE aspect can be very easy as they would "play it like Pokemon", i.e. they would choose fire over grass type for easier battles while disregarding the effects it has on their level up.
For PvP they would also be able to match up with other casuals in their own league, if they then would want to try something more competitive we have some other things in mind where they won't have to rely on their "wrongly trained monsters" alone. This could act as a bridge where if they get interested in competitive battling they would then have to start leveling new creatures they've caught in the "correct" way which is harder, more time-consuming and more resource-consuming.