Act like a Trigger only with specific object

Well... I've been searching, but I haven't found what I need.

Ok, 2D platform game. I want the enemies collide with the ground, therefore , I can't make them Tiggers(because they falling forever and ever) , but i want they act like triggers when collide with the player.( only with the player)

"IgnoreCollision" doesn't work, because I don't want they be ignored by the Player. I need the enemy act like a trigger to change to true a boolean in the script of the player when collide with an enemy. ( with OnTriggerEnter2D ), and "IgnoreCollision" directly stops paying attention to the collision.

I really dont know how to do it. :(

Ideas? Thanks! ^^