If a creature has no available adjacent slot, then it won't move at all and only take damage.
Technically flooded slots are still "available". They have a field effect present, but you can still play creatures there. Better put that specific exception into the OP.
Even with that point not withstanding, all you have to do is fill their middle row with boulders using this spell first, then play flooding.
If they have anymore than 3 creatures, that is not going to be very hard.
For this card, adjacency would definitely need to stay "to the sides" only, otherwise the above scenario will be too easy to pull off.