I decided to try this challenge earlier today, but my fastest time so far was around 21 minutes, even though I did take some advice from other posters in this thread. (My game's version was 2.3.7, and the AI opponent was the default game AI by the way.)
I started things off by ordering all 3 of my starting factories to produce both Trucks and Combat Engineers (around 2 trucks from my T1 factory, 3 trucks from my T2 factory, and 3 engineers from my cyborg factory at 1st) so that I could quickly take the central plateau. Once those were done, I then started producing a few Commander Viper Wheels (You start with a command relay center by the way.) to bolster my forces and reduce the amount of micromanagement that I'd otherwise need to control my forces (by automatically telling my units to go for repairs as needed, then return to battle once fully repaired, and by automatically assigning units that were produced in different factories towards these commanders). Afterwards, I began to pump out endless amounts of Medium Cannon Cobra Wheels (to keep their speed high while mounting the Medium Cannons), Heavy Gunners, Grenadiers, and Cyborg Flamers, sometimes building some engineers to replace the ones that I lost. (I did lose a few trucks and engineers as they were coming under fire from scavengers mostly while I was building my initial defenses on those chokepoints.)
Initially, I sent my trucks and engineers to build a number of defenses, starting with a Heavy Machinegun Hardpoint and a Flamer Bunker on the western chokepoint (I'll call this position B) to keep the western scavengers away, then blocking off the northern chokepoint (I'll call this position A) with Light Cannon Hardpoints while also placing a couple Repair Facilities nearby (so that a large army can get repaired more quickly), then placing another Heavy Machinegun Hardpoint and Flamer Bunker on the eastern chokepoint (I'll call this position C) to keep the eastern scavengers away. Once all this was done, I then built a number of mortar pits near the northeastern oil derrick on the central plateau, and a couple sensor towers: 1 at position A (for defense) and the other at the northeasternmost edge of the plateau (for bombarding the opponent's oil derricks, as well as the scavengers' eastern oil derrick). I also built a couple Cyborg Factories on the plateau so that I could more cyborgs for assaulting the enemy base.
While I did all of this, I kept one of my trucks close to my starting base to demolish all of my base defenses there (for extra power. Besides, I didn't need them), then to build a power plant and a couple research labs (and upgrade them) afterwards. I also built oil derricks as positions were being secured by my new defenses. Later on, I built an extra power plant as I took over both of the scavengers' 2 oil derricks.
As far as research was concerned, I started things off by researching Improved Hardcrete (to help keep my defenses alive) and HEAT Cannon Shells (as I was going to use Cannons, including Medium Cannons, for both offense and defense). Once I researched Medium Cannons, I then researched Mortar Pits so that I could build those Mortar Pits at the NE oil derrick as I mentioned before. From there, I decided to focus on improving the firepower of Cannons, Mortars, and Flamers, as well as improving both vehicle and cyborg armor, throwing in the occasional Synaptic Link Data Analysis or Factory Upgrade from time to time.
Note that I had my units set to fire at long range (except for flamers, in which I set them to close in at short range) so that they can keep out of range of enemy base defenses. I also alternated between having my units retreat for repairs at medium and heavy damage so that I can keep losses to a minimum.
As for how I completed my objective, I decided to do this cautiously. My opponent had built a couple machinegun bunkers near the entrance towards its base, as well as numerous cyborg flamers and cyborg lancers to protect its base. That, and there were also all of those pre-built machinegun hardpoints and towers that needed to be dealt with before I could safely storm the main base.
Creator of Warzone 2100: Contingency
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