The Archanist Mobile Anti-Air Gun is a UEF land unit. It is also an anti-air unit. It can be upgraded with shields and a direct fire cannon. It fires in bursts which can easily knock a plane out of the sky.
Consider adding a few of these to an early game strike group to provide anti air support.
The Archanist is the cheapest unit in the game, tied with the Illuminate Crahdow.
It's worth noting that once you have the Linked Railgun research, your Rock Heads will have an AA capability only slightly weaker to the Archanist, superior firepower against land units, and much more health. The Titan and Meteor will have roughly the same AA capability, and also more ground damage. Finally, the Demolisher Artillery actually has more AA capability, 60 for the Demolisher compared to 44 for the Archanist and Titan. The AA of these upgraded units makes the Archanist mostly obsolete. But do note that the Archanist is considerably cheaper and quicker to build than the other land units, making it useful for almost instantly making a highly cheap army or gaining factory veterancy.
Consider sending in Archanist only when ground attack capability is not necessary. For example, when guarding an island reachable only via aircraft, land defense isn't necessary, so the Archanist is the perfect unit for the job.
Consider 15-20 of these to guard your base throughout the game if you do not have the resources for Anti-Air turrets, which cost 250 mass each. These Archanists are cheap to build, even cheaper than Wasp air-to-air figters, and any other air-to-air units, like gunships. They can also be seperated from a regular army to guard the base while you are scouting or going for a final run to the enemy base.
Unfortunately, these units cannot be upgraded through research to have very good ground attack capability making them almost defensless against Brackmans, Yenzoos or even other Rock Heads. They are only good if the enemy uses air as their strategy, and serve no purpose other than to earn factory veterancy if the enemy uses land, considering it takes a lot of them to take out a Transport.