For physical strength:
* Kamzor (कमजोर) - weak
* Nirdbal (निर्बल) - feeble
* Durbhal (दुर्बल) - frail
For mental strength:
* Kamjor (कमजोर) - weak-willed
* Nirbal (निर्बल) - irresolute
* Durbhal (दुर्बल) - timid
For general strength:
* Nirdbal (निर्बल) - powerless
* Abal (अबल) - helpless
* Shaktiheen (शक्तिहीन) - powerless
The best opposite word for "balwan" will depend on the specific situation and the intended meaning.