a) Learn how to Hotkey!
Hotkeys are vital to aiding you getting through the final dungeon quickly. Here are some useful ones:
The Number Keys will select the corresponding squad for immediate movement. For example, pressing '1' will immediately select squad 1 on the map, ready for movement. It's really a whole lot faster than scrolling over to the squad you want to move, clicking on it, then clicking 'move', THEN moving it. It can cut quite a bit time off of the final mission.
The E button will immediately end your turn, so you don't need to click, go to the menu, then click the 'end turn' button to end your turn. This can also really cut down on the time you take to win the mission.
Just using these two easy shortcuts can easily cut off 25% of the time it takes for you to finish the mission
b) Split up your squads!
Another good idea is to split up your squads, sending two to each side of the map (So two can be sent to take out Mitosemar, and hit the seals above her, and the other two to take out Medario and Ntushitora, and get the magic seals on that side of the map). Really, sending all four squads together is completely unnecessary to win the mission. You can easily take out each side of the map with just two squads. Just make sure that you have strong characters in both groups of squads.
c) Save Often!
Trust me, it REALLY saves you quite a bit of trouble if you save often during this mission. If one of your characters dies, it is far easier to reload and get back to where you were before if you save often.
Those are my tips. Feel free to post your own over here as well!
Hope that helped you out with the worst mission in the game!