Note that the Khralean Special Ability Unit (the Infector) can convert your Marines, and also infect your troops. While the poison damage of one per turn may not seem like a lot, it can severely cripple the power of a Helicopter, as they only repair at one hit point per turn.
The Khraleans possess an advantage in that they can generate a relatively large number of fast moving troops (especially the Underling and the Swarmer) relatively early in the game, but don't let this fear you as Underlings are hopelessly weak to Helicopters and Swarmers do little damage against them and and also receive significant damage from Marines, especially in advantageous terrain. Marauders will do significant damage to them and run back behind healthier units for repairs to fight another day after attacking.
The best mix of Sapien troops to fight the Khralean hordes is a combination of Marines and Helicopters (and some Marauders as well). But watch out, a Garuda is only 350 credits compared to a 500 credit Helicopter, and most of the time they will deal equal damage to each other. Use strong positioning to protect the Helicopters and if possible maneuver your Marines onto Forest or Mountain tiles to inflict severe damage onto Khralean aerial units.
It is very rare that a Tank should ever be built against Khraleans, as it has zero capability to damage aerial units such as Swarmers and Garuda. If your Khralean opponent has built a Pinzer the correct response is usually to overwhelm the Pinzer with repeated attacks from Helicopters, fully utilizing the gang up bonus.