Presently there are some differences in rule sets but if you are allowed to clinch with one hand see below.
Try and pull their head down with your lead hand hooked around the back of their neck. Uppercut them repeatedly to the chin or or nose then when they shield go to the body - floating ribs or solar plexus. After 2/3 shots wheel them around in a semi circle to the side of your lead neck gripping hand as it keeps them slightly off balance making it harder for them to defend.
You must avoid this yourself at all costs by shrugging their pulling arm off with your shoulder