Remember that although building permanent walls by stacking C-cells is helpful, the chain reaction caused by a well-placed stack on top of a lowlier cell type can completely change the game state in your favour by flipping a lot of enemy cells to your colour.
It's tempting to use your more powerful cells as soon as they become available, in order to maximise their usage. But the timing is vital: there's no point transforming the board with a C-cell if your opponent can just wipe out your gains with a Virus. Sometimes, saving them is the right thing to do.

Because multiplying cells can only flip next-door neighbours, it's often a good tactic to try and grow into spaces that don't lead to your being adjacent to the enemy. Then, when they grow into a space next-door, you can unleash a transforming wave onto them and rob them of the chance to respond.