The only real counter for mortar pit suggested here is the Mortar HT. If you think the only counter for 400 MP nondoctrinal unit is a doctrinal unit, you are already admitting that Mortar pit is OP.
You had been proven yesterday during 2nd or 3rd game of the day, where pit didn't even lasted 5 minutes and MHR was uninvolved.
MHT is not mandatory, it just makes it trivial.