I took another stab at this and it turns out these missions on TCO still a few notches over the top. While you can now actually start the mission because the towers won't immediately slaughter your base, the actual mission is still rather difficult. The main problem I'm having is how enemies have LUDICROUS range. Missed shots from them shooting at the useless allies fly for _several screens_ and then hit the base for extremely high damage because of TCO difficulty. The damage adds up very quickly and in order to protect the base from this spam you essentially have to hold 40% of the map at all times, which is not possible when you have to go back to heal (and you will have to heal quite often considering the high damage towers deal along with the constant stream of enemies).
This isn't entirely a battlefield problem as a general problem, but enemies really have ludicrous range. Their projectiles should not fly nearly that far. It is okay if they outrange the player by quite a bit, but not like by three times (or more!) as much range. If you have some reason to keep the crazy range, at least give the battlefield base some sort of radar dampening effect, where enemies have to be at least within a somewhat reasonable distance to damage it.
In addition, the battlefield allies really are useless on TCO. Even a single enemy solos every dude on the battlefield while I heal, or would if several more weren't constantly being spawned to help the one enemy out anyway. The best I can ask of the allies is to absorb a hit once in a while but they also act as targets for the enemy to shoot at, which means more projectiles are spawned than if I were alone anyway, since I use a cloak of shadows at all times.
Oh and the mission seems /possible/ but... Well, unless you have an AMAZING dodging run (or just have a glass cannon and blitz everything) and win quickly, it'll end up being a long, drawn-out battle. Which is fine, but you always lose because of the mentioned long-range spam chip damage issue combined with the allies being unable to hold the enemies off whatsoever while you heal. If you yourself constantly hold these enemies off, you will never get a chance to heal for a long enough time and will probably end up dying eventually.