In multi-player mode with two or more CPU enemies, ally with the strongest one. Then, press [Ctrl] and fire at them. They will not retaliate. You can also build a Tanya and use the same strategy on their buildings.
Tell the vehicle to move onto your repair facility and keep your sell cursor on the vehicle, the sell icon will turn green and click when it does so. If done correctly then you will have sold the unit and got that extra bit of cash to build that mammoth tank.
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