Making an accurate trap becomes harder the smaller and more agile a creature is. Especially one as chaotic as the Gremlin, inside of a house they have so any avenues of approach it's not even funny. If they were like dogs and could only walk/run would be one thing, but with their abilities to climb and everything else on top of intelligence they would be very difficult to hinder.
Humans with all of their intelligence, are going to usually go in a pre programmed route to get to something you know they will take the stairs, doors, windowes etc.
Cmon Riv, do you even ambush tactics bro? Just kidding. I hope this makes sense to everybody.
20 is a lot, even if they just straight bullrush in a single file column, probably ly half are going to get to Kevin. Remember, his traps are a one trick pony, and they are going to hinder Kevin's ability to move around the house and evade the Gremlins as well. O top of them being tough little bastardized and most of his tricks being non lethal, I see very little chance of him winning this scenario.