Ok. Though Advance Wars has many characters with different skills there is one thing that will always stay the same. Follow your characters strengths. CO Powers can turn the tide in any battle, but if they are used ineffectively they are only a waste. You can look at the basic strategies for each character in the game.
Andy
His CO Powers allow him to keep his units in the field for a much longer period of time, and are great counters to the damage dealing CO Powers of Hawke, Drake, and Olaf. Because of his balanced forces and his healing CO Powers, Andy prefers a large, balanced map with numerous bases, airports, and naval ports.
Olaf
His powers should be used very strategically, at a crucial point to either halt the opponents' retreat or advance, allowing Olaf to take the initiative. Olaf's strategies greatly resemble those of the Soviet Army in World War 2, relying on 'General Winter' (a very cold winter with many blizzards) to halt the Nazi Army's advance on Stalingrad and Moscow.
Grit
In close-quarters, Grit's CO Powers can allow your indirect units to massacre a very large portion of the map (or possibly the entire map if you're playing in a small, enclosed map like Little Island). After using his power, artillery units fire as far (or farther than) other COs' rocket units! Rockets for the price of artillery! You'll find the sound of artillery fire will shortly become your halleluiah chorus. You'll want a good proportion of your army to consist of indirect attacking units such as artillery, rockets and battleships. However, don't let that 20% direct attack penalty scare you; you're still going to need a couple of tanks to clean up what your rockets don't quite finish, or a couple anti-air to cover your rear whenever airports are involved in the match.
Colin
Colin is best used on maps with lots of properties and with the fund rating set to about 6000-9500 as his SCO Power uses his cash to increase his units’ atk power all the way to horrific amounts with up to a massive 999% atk which would absolutely devastate your foes. Colin's CO Power is useful for earning up cash to deploy his cheap units to flood the enemy, and when the time is right, activate his SCO Power to crush your horrified enemies.