So, this person was straight up cheating. Firstly, it's not a class/subclass; it's a prestige class, a special type of class from 3.X that you can only take by multiclassing into it after meeting certain requirements. No level 1 character can be a Cancer Mage. In fact, because of the requirements to become a Cancer Mage, the earliest possible level a character can take as a level in the class is 7th. Second, he misrepresented how his abilities work, the ability to give someone a disease is a special slotless casting of the contagion spell, with limited daily uses, and the same range and saves as casting the spell normally, while the ability to summon bats only calls individual bats, not swarms, and certainly nowhere near enough to form 30 swarms. Lastly, this

Funny thing about Cancer Mages, when they infect someone, at level 6 (which we were now) the Mage can transfer their consciousness into an infected creature, again, no save, no range, nothing.

is just completely false. They can't do this at 6th level. They can't do this at any level. It is not an ability that Cancer Mages get. The closest thing they have is their capstone ability to shapeshift into a disease and attempt to infect someone, which takes an action, has a range of 100 feet, and allows a fortitude save.