Dragoons are fairly strong Protoss units capable of dealing with air and ground enemies. Use dragoons in large groups of 6 or more and upgrade the singulatiry charge. An attack with mass dragoons can be devastating but siege tanks, horders of zerglings and lots of marines and medics can pose a problem.

Reavers are slow, use shuttles to carry them around in the map and unload them to unleash their dangerous scarab drones. Upgrade shuttle speed and scarab damage. Use reavers in pairs inside a shuttle for added survivability.

To combine reavers and dragoons in an attack. A squad of 12 to 20 dragoons and 2 shuttles with reavers are enough to eliminate a base of average defense. This is mainly a frontal attack. Attack with the dragoons first, then drop the reavers right behind them. The dragoons will blast the enemies protecting the reavers at the same time. The reavers, continue releasing the scarabs to decimate the enemy.

For zerg with lots of sunkens and hydras, this is a good strategy. Do not use it if zerg go for mass mutalisks and guardians which can overwhelm your dragoons. Terrans are easily beaten with this type of attack just be careful there are not too many sige tanks out of your dragoon or reaver range.

Build Order of Dragoons & Reavers