Assuming you are conserving points to purchase a unit after your first flag capture, on Hard you can reasonably purchase 3 squads, but on Heroic, only 2. This is more feasible on Hard than Heroic, but purchasing enough Infantry Squads will allow your team to find a more tactical solution to the first flag. Infantry, when used en masse, is one possible way to capture the first flag. Also, the mortar is on average slow to kill its targets which makes the mortar very inefficient, because speed is very important when it comes to capturing the first flag. At 275 points, a single mortar will be costing you nearly half, or more than half of your mp total at mission start. However, on Hard and Heroic you will only receive 600 and 500 points respectively. On Normal difficulty, it is not too big of a deal, as you have 800 mp to spend on units. While this is not a wrong choice, it is a poor choice. All too often players will purchase a mortar as a means to take out the first line of flags. There are many ways to deal with infantry early-game. Additionally, you will also want to conserve roughly 150-200 points so that you can purchase some early reinforcements to help you clear out the rest of the first line in good order. In a standard skirmish, every force needs at least 3 things at the beginning of a match: a medic, a way to deal with large amounts of infantry, and a way to deal with light armor. This being said, certain consderations must be taken in order to capture the first line both quickly and efficiently.
These are the units that will be fighting for the first line, and will also have a large influence on setting the pacing for much of the game. The initial purchase of units is often the most overlooked, and also one of the most important parts of a skirmish.