Make the sensor self switch page just an "approach player" movement, and a trigger set to "event touch". After that make the event turn on a different self switch that isn't being used by the enemy, for say "B", and have it turn it on. This way the event will turn on another switch when the player touched it.

after that is set up, make the page that is turned on by the self switch you established. This page will have conditional branches that will tell the event weather or not to attack the player.

I'll throw together a demo and send it to you, i think it will make more sense on what im trying to say.

Demo post below contains the download...