The Angel was originally only available to AirMech Prime owners in Beta 21163. Now it is available to all players who own either a variant or the base AirMech. The Angel is the second slowest AirMech after Paladin. The Angel's ground weapons are extremely weak but it is able to do high burst damage with its Sniper. The Angel experiences a large power spike from level 7 and beyond once its ultimate ability has been leveled at least once.
For an Angel you don't rely on units much but good units are still a lot better than terrible units, so for the infantry you bring Soldiers. For the tanks you bring a Longhorn/Gemini/Dillo for early and mid game and Goliaths/Devastators (though I would only recommend Devastators for Survival mode). Use a Seeker or HAAT for anti-air and a Ratchet for support. Lexi is a great pilot for the Angel but Mako can also be good. Use Energy Guardians because the Sniper Rifle takes a lot of energy. My personal favorite item for the Angel is the Fluorescent Lightbulb because when you are low on energy you can make a better escape.
The Sniper Rifle is the Angel's most powerful attack. However, its firepower is severely limited due to large amounts of energy consumption per shot and the auto-lock will work against you when the ability is targeting units that you were not intending to lock on to, thus allowing an enemy AirMech to catch you off guard.
The Loop ability can be used to dodge incoming missiles, but be careful as it has a long cooldown. Glide is also useful for dodging oncoming weapons fire (heat-seeking missiles are immune) and if used correctly can cause your opponent to focus on the battle rather than assaulting outposts.
It can be very frustrating to deal with an Angel sniping your units from afar. Take advantage of some key elements of the Angel to make life a little easier.
Consider packing a light unit, such as Jackal, Armadillo, or even Zipper. When forming an army, place your light units in front of your more expensive Heavier units. The auto-lock of the Angel's Sniper Rifle will lock onto your light units in front, causing the Angel to waste its energy taking out the light unit. You likely will lose many light units, but only at a marginal cost in relation to your heavier, more important units.
Angel is a very weak fighter up close. Pick up some units, fly over to where the Angel is sniping from, drop them at different angles to the enemy, but don't transform to attack the enemy. Let your units deal damage to the Mech, and move them whenever the Angel aims its Sniper at one of them. After a short time, the enemy Angel will have to transform to escape. Now that you are already in air mode, you will have a very easy time taking the Angel down before it can escape. This process is typically easier with anti-air units.
Angels are very outpost-dependent, as they will need to fall back to their outposts to recharge their Sniper Rifle every so often. Use the moments when the enemy is recharging to attack unguarded positions.