Nerf giga-cannon

You need to retrieve to wait until it cools down and then all rush at once. 6 bike missiles would kill it.

Use anti-vehicle aircraft?

I really enjoy the Giga Cannon, but I don't use it enough; I always stick with the tank, ugh fun builds just aren't a thing in this competitive game.  ...That's why "Unranked" was a breath of fresh & fun air.

You try to counter MRLS too with a bike rush 1 at a time?

Don't counter a giga with bikes, they are weak to air, infantry and if they aren't tile blocking then two pitbulls or one tank will kill it. Stealth tanks or orcas are perfect. Don't know why you would complain about a giga when a disruptor kills a base in a few seconds and mg squads just got buffed so there's only a few units that stand any chance whatsoever. 

They aren't op, you're just countering poorly. 

Since this topic was posted, I've replaced the Scorpion Tank in my deck with the Giga Cannon, and it's situational a much more fun and triggering unit to bring onto the battlefield; especially when buffed with the +attackspeed/movespeed from a general power.

This was in the beginng of a match he rushed them and used the oxanna buff on them i had no time to changr builds

If a tank (predator/scorpion/tick) manages to get close to the giga cannon before it sets up to attack, the tank easily wins. I often do this with Tick tanks, having a scout nearby to see if they're about to attack the tank, and the moment it moves forward, I order the tank to move forward, destroying it with little to no damage for said tank.