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.

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?...

I really detest this game. It's grindy, the guns have the most f*cked up priority system you could imagine (let's shoot enemies who are closest to me rather than closest to the keep) and it's grindy, forcing you to replay every level on every difficulty to get unlocks to even have a chance. Just ignore this game....