best strategy:
1. get 2 magnets full sell guns and buy 2 shockers
2. get 2 missiles then upgrade shockers to max
3. get more missiles, sell magnets and max everything
4. arrange them to your liking :D
5. dont move and keep enemies from even becoming visible.
WIN
Another effective way to hit ninjas is with scorching aura (temple). All the temple needs is one target and it will hit everything in radius regardless of stealth, and as combined with the extra hit upgrade tactic can be a true time saver. This reason is why i recommend getting 25% of demon first and then moving to crypt. saves a lot of frustration in getting the 5 brilliants.
 "UPGRADE COMPLETE!"){kind=icon}
 "Cursed Treasure: Don't Touch My Gems!"){kind=icon}