Behind the challenger's initial position is an area surrounded by cliffs. It is a good idea to deploy MCV here. This will let the enemy launch attacks from only one direction, reducing the difficulty of defense.
When the Blasticate is not active, the Scorpion Cell challenger can use Tyrants to launch a raid directly behind the enemy base. This feature also allows the Epsilon challenger to send the Infiltrator directly through the Driller to the enemy base. In addition, Scorpion Cell Challengers can use the Oxidizer & Gatling Tank tactics to deal with Harbingers. If they are crafty enough to get their hands around Salamanders (be careful of Huntresses though as they are not fooled by disguised infantry), they can effectively shut down any airborne attack on their own.
For Chinese challengers, it is also a good idea to use the Iron Curtain-protected Centurion Siege Crawler to attack enemy bases. If another challenger also uses the Soviet sub-faction, a second iron curtain protection can be applied to the Centurion.
Last Bastion is also a good choice. The challengers can build Signal Inhibitors to defend against Harbingers attacks and assemble as many Godsbane and Sweepers as possible after establishing a base behind the initial position. However, Haihead is not recommended due to their lack of defense ability.
