I've just completed Rescue the Ally IX so I might be able to provide you some help on that. Note that I had very high tech and generals when I did it: art 11, tank 10 / Guderian, Patton, Govorov, Manstein, Zhukov, Eisenhower (all marshal). So try my strategy a several times and if you still feel it is impossible you'll have to inprove your strength first.
Here's my lineup:
Guderian: Super tank
Patton: Heavy tank
Govorov: Field artillery
Zhukov: Batillion artillery
Mansetin: Armored car (1st build)
Eisenhower: Armored car (2nd build)
Infantries: dummies
You build armor cars for two reasons: 1) you have low income under time pressure 2) you need mobility to reach to your ally fast. Don't assign general on the infantries. I've tried it a few times and it is a waste of general.
Strategy:
Your aim is to rescue your ally. Letting him die and taking over city yourself might sound like a good idea but it is not in this mission. The enemy forces are very large in number and the ally general is infantry, which is weak in health. You will never find a chance to take the city because killing the ally and taking the city are done at the same time without fail.
Here are the strategies you should follow instead:
1. Avoid the towers. To be more exact, don't waste any firepower on them. None of them is a threat to your ally and they don't produce any aliens, so they can be ignored.
2. Destroy and occupy all the alien cities as soon as possible. This is the key to this mission: you need to cut their factories so no more reinforcement comes.
3. When fighting near your ally, try to create a formation that blockades the ally from the enemies, while maintaining maximum output (plain fighting, range).
So, your route should be: from the starting position straight to west ignoring the tower, one group attacks the northern city while the other heads to the ally position to defend him. The group in north should destroy the enemies there (except tower) as quickly as possible and head straight down to regroup and defend the ally. The new armored cars should head straight there as well.
More detailed account:
In turn one, you might be tempted to move Guderian to the plain hex, but don't. Just move him as far left as he can go and attack the UFO there. If you don't do this the UFO in the south will come up and block Guderian from moving west. You can also restart for blitzkrieg skill.
Here's the part I don't remember, so try both:
1. Patton, Zhukov and Govorov destroy the UFO first at cost of delayed travel to west.
2. They just head straight to west even if it means not being able to attack the UFO.
After that, proceed naturally following the strategies. You'll find it easy to capture the northern city because it is occupied by UFO not carrier. Also, the tower always attack Govorov so expect him to take one-two shots. It cannot be avoided. Use the two infantries as dummies.
