I don't know about that. It's Kachar, are they in the Indian tech group?
Edit : okay, there is definitely a bug somewhere, though I can't find where.
I have built my army back thanks to my 35 000 manpower and 15 000 mercenaries. My new 25 000 men army won the battle against the said 17 000 "terminator stack" without any problem now.
I'm steamrolling Kachar as I should have been able to do since the beginning.
Strange...
I checked, they have a better unit (indian arquebusiers). But still, now i'm winning again without any problem. I am far from the "two day battle where they have 0 loss and you lost 20 000 men".
And nothing has changed in my stats since the first defeat (morale, tech, maintenance, general).
I won't be able to post my save for bug report :/. I have advanced since the bug and I forgot that ironman saves are always overwritten automatically...