The Spy Fly is an excellent screening unit in the new Savage Swarm (insect) faction. In addition, it helps out your blasting units.
Where the Spy Fly really shines is that when used in an army lead by a Savage Swarm monster, it moves for free. This allows you to use the Flies in both an offensive manner (by leveraging it for Spotter) and defensive manner (by using it to screen your monster against power attacks).
Strategies and game play:
Ideally, you'll find room in your list for several of these guys. This allows you to make the best use of the Spy Fly. What you do is create several positions on the field where your monster might want to retreat to, ideally closing the monster in on all sides with buildings, Spy Fly, and other units. Because the Spy Fly moves for free, this should be a fairly easy tactic.
When you're intending to activate your units to make a blast attack, you'll move a Spy Fly into an offensive position. Again, since they move for free, you'll end up with even more dice to blast with, which increases the likelihood of actually hitting.
Combos with other miniatures:
During Series 4, the only serious combo available is with the grasshoppers. Since they're basically G1-Strike fighters, it is pretty easy to get serious damage when you group the grasshoppers en masse.
Ways to counteract it:
The Spy Fly suffers from low defense. It is very easy to destroy, particularly if you do not space them out. Use figures which can hit multiple miniatures by rerolling dice in play and you'll easily destroy them.
Free movement. Spotter.
Artwork and aesthetics:
The miniature if very attractive. It has big bug eyes which really stand out on the battlefield. It also has a unique color from the other Savage Swarm figures but still fits in with the overall color palette.
10. Easily the best new addition to Monsterpocalypse released in Series 4.