I tried what that video shows....does not work for me. It's easier to run around and avoid the other monsters and find the slimes.
Metal Slash does crit, but very rarely. I also found that LMS tend to stick around long enough that I could use Metal Slash with one or two characters and regular attacks from the rest and still kill them before they ran. Of course, Thunder Thrust is easier. If you don't have Thunder Thrust, equip the Falcon Blade then do Metal Slash for a possible 4 points of damage (half the LMS HP), or Falcon Slash with Falcon Blade for 4 attacks=4 points of damage and 4 attempts at Crit.
Side note, Crits don't always insta kill metal slime enemies, I've seen my mage multiple times Crit and do a single point of Damage...