If I play Kennen from hidden, can I stun an enemy at a different battlefield?

No. When you play Kennen, Keeper of Balance from Hidden, the "When you play me... stun a unit" is a play effect (Rule 383.4.a), and Rule 811.1.d.2 restricts play effects of hidden cards to target only units at the same battlefield where the card was hidden. Since "Stun a unit" has no targeting restriction that makes targeting another battlefield necessary, you must pick a target at Kennen's battlefield.

If Kennen later attacks (not from Hidden), the attack trigger is a regular triggered ability, not a play effect — so that trigger can stun a unit at any battlefield.