I've completed levels 1-14 with 3 stars on easy, normal and hard but I simply cannot survive level 15. Any tips?

Was in Groupees Casual Bundle 4

Are there any shortcut keys for this game? Eg. for pause/unpause, next wave, speed up/down, cast spells.

Okay, so I vaguely remember seeing something pop up when a weapon was introduced about how to attack hidden enemies but I don't remember. Can someone share how to make invisible enemies open for attack? Much appreciated :)

I have now 100% of the achievements, but i have only 89 of 720 possible Stars yet and have not completed the first of 4 campagnes. 60% of the achievements are possible in the first 5 missions. 4 achievements are possible to get directly easy at the first mission when first playing it. Is it planned to implement a few harder achievements?...