At the start of the campaign, your only haven will have 6 rebels, assigned equally to Supply, Intel and Recruit jobs. It's recommended to switch all of these to Intel to detect as many missions as possible. Every mission matters at this stage, as they provide both valuable resources and experience for your troops.
For the Avenger, your first facility should be the Guerrilla Tactics School, even if you can't buy the perks it offers, since it allows you to train rookies in the class of your choosing for free. As for research, any option available is good, especially Basic Research as it increases the speed of your research by one extra scientist, or rushing Laser Weapons, as the increased efficiency at killing ADVENT will help with mission completion. You will want to pick up at least one or two engineers from either missions or the Black Market, as excavations are a very good source of materials early on and you will need the space.
Researching and building Resistance Comms should also be a priority, as you will want to contact as many regions as possible early on. More regions means you can dedicate rebels to other jobs, ADVENT Strength will be less concentrated, and more missions will be available anyways, including ones that counter Dark Events, as well as Troop Maneuvers and Supply Line Raids, which provide much needed corpses.
If you don't have enough resources for weapons or comms, one of the main objectives to do is completing the Skulljack project, for which you need the ADVENT Trooper and Officer autopsies. Later on there will be many other research and Proving Grounds projects available, so researching this early will be one less mandatory task to worry about.
In the first few months of the campaign, the only main objective is to "Find a Lead". The first mission for this objective is not obvious, as it will be a "Find a Lead" Guerrilla Ops mission with Intel as reward, but once completed the Liberation missions will be able to be detected, starting with "Liberation: Rescue a Defector".
The first liberated region should be one where ADVENT Strength is low, since the missions will have a positive Strength modifier and they will cause ADVENT to raise its Strength even further. The haven for the region should also be full of rebels, as recruiting them after liberating the region is slower. Depending on your progress, between May and July are the best times to complete the first Liberation mission chain, especially if you have Magnetic Weapons available.
Rescue a Defector and Hinder ADVENT operations are standard, if harder than normal missions. Destroy Network Tower has restrictions on the size of the squad you can take, but no infiltration time limit, so make sure you bring a hacker and a team able to complete the objective quickly. After completing the first Network Tower mission, not only will the final Liberation mission be unlocked, but the ADVENT Blacksite mission will also appear, in a region as far away as possible.
For the final Liberation mission, Assault ADVENT Base, there is also no infiltration time limit, so bring your best soldier with the better gear possible and keep in mind this mission will be a long one, charge based soldiers like Technicals may run out of charges long before the mission is over. After successfully completing your first assault on a regional ADVENT HQ, the Avatar Project will be uncovered, revealing its progress. In addition the region will be fully liberated from ADVENT control, increasing the resources you get from the haven as well as preventing any non-invasion missions from generating in that region.
- Dark events are easy to detect but happen in many regions, contacting new regions is the best way to stop dark events.
- Doing missions in a region builds up "vigilance" in a region which is the hidden variable that garners Advent's attention and causes them to send supply raids and such to increase Advent Strength in a region.
- Increases to Advent Strength is retroactive to missions that are being infiltrated, thus you may get higher enemy strength by the time the mission launches than you had planned for.
- Valuable corpses, supplies and alien materials are mainly supplied in enemy materiel missions, these missions appear more often in regions with high advent strength, and reduce said strength by one, taking a harder mission by launching the mission early can give you more advanced corpses and help make future missions in the region easier
- The GTS (Guerrilla tactics school) can be rushed out, and then the "Wet Work" upgrade can be purchased to both assist with squad leader training and gaining more high ranking troops
- Advent Trooper Corpses are important only in giving the autopsy and for Alloy Plating, the rest can be sold or rendered. Last option is better in late game, when you might find yourself short on alloys and/or elerium, since it's fast and doesn't hinder your production.
- Purchasing staff is a great way to even out lopsided engineer to scientist ratio's and high level recruits available are quite cheap
- If You have revealed the Advent HQ in a region, attack it asap. Advent strength will most likely go up because of your shenanigans.