The achievement guide here lists no mission strategies, and I felt the mission guide that is linked isn't particularly helpful, so I decided to make my own guide for finishing each of the last missions of a chapter on Insanity. I'm assuming you've played to the last mission on another difficulty, and only care about the last mission. I'm also assuming you are playing co-op with 3 or 4 people.
Hopefully this is helpful to some of you. All these strategies worked great for my team. These are just possible ways to win, you could find something else that also works
Complete Chapter 2 in Insane.
The last mission of Chapter 2 is a Warfare match on Torlan. As you've played through the chapter, you should have collected two cards. One card will give you two additional members to your team; the other card will remove two members of the opponent's team. Either way you will have a 1-person advantage, but I highly recommend removing two opponents instead of padding your own team. Fewer opponents means less enemies trying to kill you, and your orb runner is less likely to be intercepted and killed. The latter point is critical. A good orb runner is the key to victory here.
The key to winning this match is a fast start. Before the mission starts, decide which member of your group is going to be the designated orb runner. This person will probably see the least combat, but their role is critical to the mission's success. As soon as the mission starts, your orb runner should grab the orb and set off for your prime node to capture it ASAP. Once this is accomplished, they should teleport back to the main base and grab the orb again, this time heading for the center node.
Everyone else should head for the Leviathan at the mission's start. Take the Leviathan (and perhaps a more mobile vehicle from the base as an escort) and head immediately for the center road node. Try to fight your way passed the node and do your best to hold the area and kill any enemies that get anywhere near the center node. This should buy enough time for your orb runner to take your prime node and get back to the main base, and possibly even run the orb to the center road node untouched, if your defense is strong enough.
With the center node captured, the orb will probably reappear at your main base. The orb runner should teleport back to the base for the orb, then take it up the right edge of the map toward the enemy prime node. This route is unlikely to have much enemy traffic, since the AI will now have to worry about its prime node and attacking the center. This way, the orb runner can sneak into the enemy prime node almost undetected, and instantly claim it.
The other players should continue with the Leviathan and any other vehicles toward the enemy prime node from the center node they just captured. Attacking players now have two objectives. Objective A is to harass the AI guarding the enemy prime node, and kill them as much as they can, so the orb runner can most easily sneak in from the opposite direction of where your fire is coming from. Objective B is to keep an eye on the center node and prevent the enemy from capturing it. Stationing the Leviathan where it can see and shoot at the enemy base and their prime node at the same time is a good way to work toward both at once. You could also sub-divide these tasks between your team, assigning some people to harass the prime node while others keep an eye on the center node. Whichever strategy you take, you want to have the longest open line of fire you can, so you can take out enemies before they even get close to your nodes.
With the enemy prime node captured, the orb runner's final duty is to grab the orb again and stay with it near the enemy prime node. This way, you're constantly healing the node if the enemy attacks it, and the enemy will be unable to steal the node away with their orb while you're around and alive. The orb runner needs to do everything in their power to defend the enemy prime node and prevent its capture until the end of the match. Order the bots on your team to defend with you, and be sure to let your teammates know if you need help.
With the enemy core now vulnerable, the attacking members of your team are free to pursue any method they choose to destroy it. What my group did was have two players stationed outside the base picking off anyone they saw, keeping the enemy largely pinned inside their base, while one person went in and assaulted the core.
As long as you keep the enemy prime node secure (or the center node, if things start to go badly for you) you can win this mission in an overtime victory if you've managed to damage the core just once. In overtime, both cores start melting down, but if you hit theirs and yours is still fine, the counter on theirs will hit 0 first, giving your team the win.