Is there a way I can set an enemy to gain resistance after being hit with attacks of the same type? (i.e. after 3 fire attacks the enemy now takes half damage from fire)?
Alright, you need to setup a troop event for that and that needs to check some variables for that.
On skill tab on your fire skill put that into formula before the damage:
$gameVariables.setValue(21, $gameVariables.value(21)+1);
Change the 21 to what your fire variable will be.
Then add a state to change element resist.
Back to troop events, make one event page like that Conditions: Turn End - Span: Turn
on the bottom: if the enemy has 3> fire variable
add state Fire Resist to enemy.
After the battle ends set those element variables to 0 (I recommend using a common event to reset them). I hope this helps.