Mirza Ghulam Ahmad was a raving lunatic.  He would issue his death threats randomly and under British protection.  In this specific case, we have caught MGA giving out some rules for his twisted Mubahilas.  Firstly, Mubahila is not a way for any Muslim to act with another Muslim. In the 1400 years after Muhammad (saw)…there were scarcely any Mubahilas between Muslims and non-muslims.  MGA seems to have opened this topic and deviated from Islam.  Nonetheless, enjoy the quote.

The quote
“Let it be clear to the readers that Mr. Abdul Haqq had asked for a muba¯hila. But I cannot understand how a muba¯hila could be permissible regarding those matters of difference which do not make either party into a ka¯fir or an unjust one. It is clear from the Holy Quran that in a muba¯hila each party must believe that the party opposite is a liar, i.e., is deliberately deviating
from the truth, and is not merely mistaken, so that each side is able to say: ‘May the curse of Allah be upon the liars!’ Now if Mr. Abdul Haqq considers me to be a liar due to his wrong
judgment, I do not call him a liar but believe him to be in error, and it is not allowable to curse a Muslim who is merely in error.” (Iza¯la Auha¯m, p. 637).