I completed stuttgart and jumped into colegne. With the experience of Munich and stuttgart, i expected colegne a piece of cake. I avoided to read manual on scrin(too much easy early units).
But when warships start attacking my base and i have to crank rig four times along with mammoth in a suitable position, just to destroy a single devastator. Only one pair of mammoth and rig remained. Then i trained my firehawks and missile squads to balance the even.
I am still defending my base with my mammoth and rig now holding up.

Oh God, i have to read the manual on scrin now.

Mammoth are tough and CAN attack airplanes but are not super because they cost a lot and you need a lot to take down a scrin ship. Instead try to to have just a couple of mammoth and build full of APC loaded with Rocket infantry. They move fast and they are cheap so you can make a lot of them. If you repair them after each attack they will become Hero and then, they are very effective.