Sorry you're not finding it enough of a challenge. The AI will never be as good as a human player, but we do try to have it give you a run for your money.
The AI in OOBP is dynamic at the tactical level, but scripted at high level strategy. The scenario designer sets ups teams of units and gives each of those teams high level objectives. These objectives are changed by high-level triggers that are also scripted.
At the tactical level, the AI is completely dynamic, and teams can and do dynamically support each other.
We're currently working on a Skirmish mode, which will introduce a more dynamic strategic AI.
Also note that currently, the AI does not purchase new units. Instead, as you've noticed, it starts with on-map reserves which are not committed until certain in-game triggers. This gives the scenario designer a bit more control over the possible flows of battle. We generally think it would be unfair for the AI to throw all of it's units at the player in one go, as these extra units will only come to the player as the player purchases them.
This is something else we are changing for Skirmish AI.
< Message edited by Myrddraal -- 12/23/2015 9:21:21 AM >