Iria Zeiram The Animation [OVA]

Iria and her brother Gren, a pair of young bounty hunters, were dispatched on a standard rescue mission, but what they found was more terrifying than anything they could have imagined. A bizarre alien was secretly being imported for clandestine uses, and now it is loose to wreak havoc on the unknowing populace.
How can a novice bounty hunter possibly defeat an unstopable beast like Zeiram?