This is a little complicated so i hope the structure is clear.
Batman, Constantine, L, Light, and Lelouch are in an advisory role here. They have constant access to their charges via telepathy.
The fighters, Nami and Sakura are in their current versions, with access to all their tools and summons.
Location is London city. The characters start the battle 2KM away from each other. No idea of each others location.
Batman/Nami Vs Sakura/Lelouch/Constantine
Same as fight 1, except either Lelouch or Constantine or both are replaced with Light and or L
Same as fight 1 and 2, only the advisers can enter the fight if the battle exceeds 30 minutes (Lelouch gets his mech)
Same as fight 1, 2 and 3, only the advisers switch combatants
SCENARIO 1: No Prep or Knowledge for anyone
SCENARIO 2: 5 Hours prep. All characters have knowledge of all universes involved but no knowledge of the characters, i.e, constantine knows all about the one piece world but not who Nami is.
SCENARIO 3: Everyone knows everything about everyone. 1 hour prep. (I don't know if this scenario is redundant)
SCENARIO 4: If things get too hairy, batman can summon a non powered character from either DC or marvel to assist him in the advisory role or Nami on the battle field.