Several things change in Legendary difficulty. You do not get extra execution bonuses, and you will not have an obvious signal before an enemy launches a heavy attack. Flaws in the combat system also are more noticeable, as you can't afford to make mistakes. You must master the system and make use of everything it has to offer, even aspects you didn't need to use on lower difficulties.
Do not try to play the game on Legendary until you have almost all of Marius's executions and abilities unlocked. Since your execution bonuses will be much smaller, you need every bit.
Master your ability to dodge and block enemy attacks. You need high combos to defeat enemies, and you can only take about four or five hits. Damage executions can be quite effective.
You will face many more heavy attacks than you did on lower difficulties. Keep an eye out for enemies about to use a heavy attack and either dodge the attack, or block it at just the right time.
You can use heavy attacks of your own when you hold down X or Y. This will make combat easier for you, especially against more difficult enemies.
The blocking system is also very effective against bosses. Once you practice, it will become second nature. If you block a boss's heavy attack, the boss will be thrown off balance, which provides a good opportunity for you to use your own heavy attacks.
When you are facing multiple enemies at once, vary your attacks between them. If you try to focus on just one, the others can attack you. If you attack each of them, you can hold them off and make your way to victory.
