For random spawning (and if you have some money) I reccommend you Yanfly Event Spawner.
To make an event chase the player, set its autonomous movement to "Approach" type
To trigger the event (and begin the battle) when it chases the player, select "Event Touch" on trigger
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To make the attack from the back I suggest you to make your event having 2 states (using a common switch)
- An "idle" State by default where the event movement is "Random" and the trigger should be "Player Touch", this is the attack from the back.
- And a "chasing" state (maybe common event C) where the movement is "Approach" and the trigger is "Event Touch", this is the normal battle.
AND...
I think you will need to make 2 troops for every enemy, one for front, and other for back attack where you could force the enemy to wait on the turn 0.