hey man, nice game you have!
i'm played it and is running smooth, but the sections i have fps drop was the ones with A LOT of effects + colision checks
i'm on a AMD quad-core 3,80 GHz 8GB RAM with a NVIDIA Geforce 8400GS
i think your problem may be the JUST effects + animated backgrounds
your game is not just heavy animated; it is VERY heavy animated with heavy effects + alot of background heavy animations
i'm pretty much impressed that construct2 could handle the numeber of effects in your game without break the rapid gameplay feelling, but i think you should review your design on the fight rooms because there is too much stuff happening at the same time, maybe changing the background effects a little when there is colision checks or even removing some stuff; like, when you have a big fight you could make it on a new layout room with less effects happening on the background or change the way your dinamic effects to something simple like frame by frame or using spriter
maybe just ashley can say what is really the problem but i hope you can keep the scope of your project because is looking very cool :D