So uh this seems hard as balls. I don't think I "get" it yet. I don't remember the past games being this hard.
So a brief primer, interrupts are pretty useful in this game. If you see an enemy charging an attack (usually have some sort of circular red effect around their feet) if you hit them with a strong ability or even a heavy, there's a good chance you'll instantly send them into a stagger state. Stagger states can also be reached by eliminating the purple bars of enemies with interrupts, synergy attacks, or abilities that have good stagger damage. If it's a boss sent into a stagger state (standing on one foot is the indication from some enemies and/or leaning backwards like they're about to fall) this is a good time to use your extreme attacks.
Some people mistakenly wait until they're in a hard stun state to active their extreme attacks but due to the startup frames on these huge attacks, you're wasting potential damage. If you activate your extreme attacks while someone is staggered, they'll get pushed INTO the stun state (stun state makes enemies takes way more damage from attacks and extreme attacks especially). If it's just a beefier enemy that's a non-boss that you've staggered, use a synergy attack to push them from stagger into a stun state rather than an extreme, unless you can maximize your damage because you're being swarmed. Other than the offensive kit, rolls seem to have invulnerability.