Far too easy
Got to day 16 before I stopped, and by that point I had all the upgrades and nothing could get past. For games like this I'll give you the same advice I give anyone making a game like this. Defence games require variety in as many areas as possible to keep it fun and make people want to keep playing.
Instead of just making enemies have more health or do more damage have each of them have special effects. Make some crazy fast that self destruct when they get close. Make some immune to the Mini-Coil's damage. Make some only injured when they're hit with chained lightning. Let some planes drop parachute troops as it flies overhead. Maybe some enemies don't do damage but they incapacitate your coil for a couple seconds, or drain your energy.