Mobile, inexpensive, and packing a decent punch, Battle Copters (Often shortened to B-Copters) and incredibly versatile units.
B-Copters are very useful units throughout all points of the game, thanks to their incredible utility for the cost. B-Copters will deal moderate to high damage to almost all foot soldiers and light-armored ground vehicles, taking little damage from them in return. Against Md. Tanks and Neotanks, B-Copters are still viable options, dealing about as much damage as they'd take from them, while also being much cheaper units. Mega Tanks take low damage from B-Copters, but also deal low damage in return. Anti-Airs should be avoided at all costs however, as B-Copters deal minimal damage to them, and taking very high damage on the counterattack.
Against other Air units, B-Copters can deal high damage to other Copter units. They are defenseless against Plane-Type air units, so Fighters and Stealths should be avoided when possible. B-Copters fare well against sea units as well, dealing high damage to Cruisers if they can get the first strike. Most other naval units are defenseless against B-Copters.
The most common use of B-Copters is to harass enemy Tanks, Artillery, and Infantry, as they will beat these units handily, while also being able to fly into an Artillery's range without punishment. When the enemy builds Anti-Airs as a countermeasure, use the terrain to the B-Copter's advantage, utilizing forests or mountains to find spots that Anti-Airs cannot reach. One should be careful not to fly them too far ahead of any supporting units, however, as careless use of B-Copters can lead them to being surrounded and unable to break free before Anti-Air takes them out. One should also be mindful of other B-Copters as well, as their inability to use cover leads to the attacker dealing much more damage than the defender in Copter match ups.
The only two cost-efficient units readily available to take out B-Copters, for the most part, are Anti-Airs and other B-Copters. Since Anti-Airs are less mobile units, they are much better used as a deterrent rather than an aggressor, ensuring that and B-Copter attacks on your other units are met with immediate retaliatory attacks from the Anti-Air. B-Copters are a straight counter to Tanks, but a highly cost-inefficient one if the B-Copter is shot down after one attack. If you are lacking in readily available Anti-Air, you may need to fall back a bit to prevent any free attacks from the enemy. If your enemy is being reckless with B-Copters, they may fly too far ahead of its supporting units. If this happens, it can prove valuable to surround the Copter with Infantry, preventing it from moving until Anti-Air arrives.
After sustaining a few attacks, a B-Copter's HP will generally degrade over time. Unlike Tanks, B-Copters are much less capable of finding a repair spot, usually having to fall back a considerable amount to find a friendly airport. If an enemy B-Copter's HP is beginning to deteriorate, Infantry can prove effective at whittling down its HP even further without taking too many losses, greatly hindering the combat viability of the Copter.
As the game progresses, Stealths and Fighters may begin to see play, both of which able to do high amounts of damage to B-Copters without fear of counterattack. Buying either of these units Solely to combat B-Copters is very cost-inefficient, however.