After the battle, one of the sisters is killed, in which the other cries that she was her other half. Furious, she promises that Chuck will never get out of Fortune City alive. She then takes a dagger and commits suicide.
Knife Gloves (Bowie Knife + Boxing Gloves) are also a really easy way to kill the twins. As long as Chuck only attacks either Amber or Crystal, it should kill them in 5 - 15 hits. If Chuck has more than 6 health, then he will not need to heal at all during the battle.
If one of the twins performs a sliding attack, let them begin their sword swing before attempting to dodge. The slide looks like a blind charge, but it homes in on Chuck.
Two Shotguns can be found at the gun shop in Palisades Mall and one can be found behind the counter across from the night club. Use the shotgun (in the aiming view) and target one of the twins as they each charge Chuck. It should take around 25-30 shots to kill one of them.
Take the Light Machine Gun in the cashier area from the previous case Case 3-2: Run for the Money. Alternatively, go into the middle of the Yucatan Casino and climb to the top of the Tiki Statue, where Chuck will find an LMG and a Zombrex. So with 2-3 painkiller drinks (below), and one LMG, travel to the nightclub. Before entering, be sure to grab the shotgun underneath the counter opposite from the nightclubs entrance. Immediately after the cut scene, drink one of the painkillers, then use the machine gun on either twin. Once the machine gun is out of ammo, switch to the shotgun, and drink another painkiller if needed. Chuck should be able to finish off whichever twin Chuck is focusing on with a few blasts from the shotgun.