Level 15 is dead easy, you even have a 2way mirror left unused!
Level 17, same thing: you can even do it with 1 mirror unused...
Level 20, yep, too much items
.... a 2way mirror and a 45°block
btw.... there is a version with 28 levels instead of 25:
http://www.freefunandgames.org/games/reflections/?id=68
direct link:
http://www.freefunandgames.org/games/reflections/laser.swf