level 13 if you have trouble cause they have so many more to start take the cell in the top right corner using your biggest cell ass soon as they send out first attack then wait and build once small cells reach 20-25 you can attack a grey 8 with one wave which should take it until repeat until u take the there second bigest hope that helps :)
if you miss alot or want to get criticals you need speed it is a bit repetitive but can be fun if you have trouble with missing shadow he has high peed so u need to build up your speed