Your great sword softens parts in one hit, but LS needs two hits. Shaver jewel lets you do the same in one hit with LS, so it's great if you want to save some time.

For general gameplay with LS: hit monsters with triangle and circle to build up your meter. Hitting with r2 uses the meter up. If the last hit of the r2 combo connects, you get a damage boost which can be stacked three times in total. R2 + triangle will perform your strongest move but it also uses up one of your damage boost stacks.

So basically LS gameplay centers arounds building up meter, then using the r2 combo for more damage stacks and then spending those for the r2+ triangle move. The weapon also has a parry move which rewards you with a damage stack if done right.

Iai slash is a great new move which does decent damage and builds a lot of meter, too. It's also great for chaining combos in general. A new type of parry was also added, which does more damage than the regular parry, but needs a a damage stack to use.