I completed War Drone today for $163.93 with no money spent.
It took 13 days total with a decent amount of time playing, but its actually a fun game.
Here are my tips:
Watch the ads after each round and for the free chests.
You will start a level and it may seem impossible but keep going. You will learn to anticipate the attacks and conserve ammo.
When you get to stage 10, don't finish it. Kill everything and then let the last troop beat you. Keep doing this to earn alot of money. You can replay stages but once you beat a stage, the rewards are less for repeating it. Aim for 250(25mm)/75(Hydra)/10(Hellfire) before moving onto stage 11.
Before beating stage 20(same as above), make sure you are at least 390(25mm)/90(Hydra)/28(Hellfire).
There is a 3 day VIP free trial. I started mine at Stage 15 and finished Stage 22 right as it was expiring. Just dont forget to cancel it.
My final stats were 406(25mm)/90(Hydra)/30(Hellfire).
When in doubt, pump up your 25mm. It is your best friend!