No bug, just very difficult.
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Stronger your magnet gets the easier it will be, but also I found most of the time I missed a star was from my missles hitting something just coming into screen and either the star was off screen slightly or far in a corner etc. I always keep an eye on those when going for the 100% stars.

Just look out when you are losing the streak (nice-good-excellent etc). It is shown on top of the screen. Once streak is broken, you can find out where you miss th enemy to kill.

Its not bugged I can guarantee you.
You are loosing some enemy at some point.
Keep looking to the enemy marker at top of the screen to find where you let the one scape.

I had that same problem. It turned out ther was a hidden boat to kill, which you can see hidden in the shadows of a wharf in the middle of the screen after the 3rd ground turret, or when the first helicopters show up (by then it may be too late to kill it and maintain 100%)

Thanks for all the help ... TKA is right I'm sure that was what I was missing. Got 100% now !!

Wait you have to kill everything, even things that do not attack like boats or lighthouses, right?
Because I was intentionally missing the first boats and it didn't strike me, but some screens later it did, at a spot where it could have only been another boat... hard to tell, could be clearer. Hm.

yes,it's bugged, i have a video, near the first tower, i miss no enemy and the challenge turns wrong.

Only the Insane difficulty occurs the bugs

In this level, you need to kill all standing boats with bonuses and the lighthouse. If the medal for 100% kills is crossed out in front of 3 cannons (after the lighthouse), restart the level. I went through 20 times until I understood how.

Thank you, Lex! Wiped the level right up after reading your advice. Bless you!

Originally posted by Lex:

In this level, you need to kill all standing boats with bonuses and the lighthouse. If the medal for 100% kills is crossed out in front of 3 cannons (after the lighthouse), restart the level. I went through 20 times until I understood how.

Thx a lot!