A few tips for coop.

Coop has a severe lack of flux so in general taking mercs outside of infused mercs is a mistake.

Enemies have more aoe than the regular game. Since there's extra hexite, anything that scales with units dying is vastly stronger. examples are the tempest core.

Taking flux first is usually more beneficial than capturing bases.

Due to the efficiency of buildings, building outpostst with buildings on the map is much more cost effective than fodder.

The modifiers in galactic war and equipped arsenal make the missions signficantly easier. Armor+mandragors, Range+commandos, etc. This particularly is important with the early game units.

Building a few air units early for vision helps a lot since a lot of missions are pushing up a ramp.

Don't throw away top bar energy. Try to generate a lot of it. Legion generate it by units dying. Vynthra generates it by killing things with her abiliity.

Buildings can by time to finish what you're doing. Typically they are high expo and can really slow down attack waves.

Play with your teammate as much as possible, most of the time splitting up is wrong.

If a building can't be cancelled, usually it's because there was an error building the structure before it on the tech tree. Another building usually can be cancelled and rebuilt fixing the problem.