Keep your HP up to avoid dying from Strong's powerful attacks. Try staying above 50 HP. Use PSI Rockin α or regular attacks, healing up with food or Lifeup α, until Strong is defeated.

The lowest level where you have a chance of defeating Captain Strong is around 7.

Once you take Strong down, he'll tell the officers at the southern barricade to let you through, so you can move on to Twoson.