haven't really looked at the terrains but...
Air would want a no items, fox only, final destination-esque terrain. Particularly wouldn't want any extra living things to be present during the fight
Syl would want any kind of terrain that's fairly straightforward and doesn't have any 'moving parts'. So, something final destination-esque or something bigger/more complex as long as it doesn't have anything random going on or make unexpected things happen.
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DROP THE R AND BACK AWAY!