If I hide here to help at a battlefield, can I use it to play a unit into the other battlefield on the enemy turn?
No, you cannot.
According to Rule 811.1.d.3 and RiftJudge FAQ #3476, when a hidden card causes you to play a unit, you must choose to play that unit at the same battlefield where the hidden card was hidden.
Additionally, even if Here to Help did not have that specific restriction, RiftJudge FAQ #8911 and Rule 811.1.d.2 clarify that if a hidden spell or a play effect of a hidden permanent chooses targets or locations, those choices are restricted to the battlefield where the card was hidden, unless the ability explicitly states otherwise.
Summary of Restrictions:
- Mandatory Location: Since Here to Help is played from "Hidden," the unit you play must be placed at the battlefield where Here to Help was hidden (Rule 811.1.d.3).
- Targeting/Placement Restriction: Hidden cards are restricted to their own battlefield unless their text specifically allows targeting elsewhere (Rule 811.1.d.2).
This is my interpretation based on the mechanics of Hidden cards (Rule 811). No official FAQ exists specifically for 'Here to Help's' range, but the rules for Hidden cards clearly constrain play effects to the location of the hidden card.