Put your golem and maybe a summoner guild in the front of your base. When or before the CPU enemy makes its first attack, morph perhaps 5 or more initiates into battlemages. The CPU enemy should mostly be attacking the golem and/or summoner guild, and you should be able to kill the first wave without any battlemages dying.
After the first wave, you can try to attack the CPU enemy immediately, since it will be at its weakest. Avoid towers, and you should be able to defeat many of their combat units
in detail while you continue to harvest resources and get a larger army.
Also, your mage tower should almost never be idle in the first few minutes of the game, unless you need to spend gold early (e.g. to make battlemages for rushing or to quickly make a new mage tower) - in general, it should continue sending initiates to mine gold until the local gold is almost saturated with initiates, at which point you should probably still be making more initiates to build stuff or to convert to battlemages.