Ah, I thought avoid meant "take 5ft step to not be there any more", but now I understand.
On the Prone thing, we could also think of prone as "physically out of balance; not in a good combat stance" (not an SRD quote, just a phrase from me), which I've certainly seen when a snake is thrown to the ground and has to take a second to resume a slithering/coiled posture.
But back to the action... how about let's try this:
Oeric rushes through Cecilia's space, and she ducks aside to avoid his bulk, coiling and recoiling herself while continuing her spell. Oeric lifts off into the air over the water, and turns up into the air, ending at 75n20e15z.