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How did you improve your preservation mates ?
I often use the minimap (it's not your enemy to forget it), in full screen of course, alongside with the corner unit list, so I can see every fight and retreat units in danger.
Well, I've also had that problem, I used to engage in fights I cannot win. When I stopped it, my unit preservation significantly increased. And if you are defending and see the enemy units are outnumbering your troops, just retreat. There's no point in fighting a battle that you know you won't won.
For the tank part, harass the enemy fuel cutoff very often (if you have free squads and can do it, don't take away fighting squads), and if you feel that the opponent is going to bring out a tank soon, just get a raketen (and if you see it, and fear raketen is not enough just get a schreck for the spio).
These are basic stuff, but maybe a strategist can give you much better tips