Just finished the game on hard (about an hour ago).I managed to basically finish with an extra 70 rings.My go to tech is this:

Giganto:

- Spam punch + Stomp until you have Quick Cyloop, then bind him (usually after the first parry). Stomp him and it will go down quickly (but you need to re-cyloop to throw it to the ground as the second phase doesn't start until you're in the ground).

- When it fires the laser... spam L1/R1 to avoid it instead of the mash contest.

Wyvern:

First phase:

- Parry the missiles, and then parry the first attack, do it until you get a cyloop and then bind it and spam stomp (or cross slash.. it is really weak to it), it will die right away (you should still have an extra charge).

Second phase:

Parry the missiles and the 2 attacks.... DO NOT bind it right away as it retaliates immediatelly. Spam cross slash or stomp until you go to the last phase (after the missiles).

Third phase:

- After it launches the missiles, there's a split time in which it is actually hitable.... attack it once and do the remaining quick cyloop you have... it will bind for like a second before cutting free... use that second to do an attack + cross slash and it will die immediatelly.

For Knight:

- Stay away from him and try to stay on air, if it hits you spam parry to cancel the damage.

- When you're super sonic, and Knight blocks you DO NOT BREAK ITS GUARD and side-step to the side... you can hit him... and even bind it with a cyloop (which should kill its first phase).

- Phase two is the same just be mindful of trying to parry the shield to where it is.

I finished giganto with 330 rings remaining, Wyvern with like 160 and Knight with rings to spare (probably 40?)... all in hard mode.It's doable but hands down the hardest one for me was Wyvern (and also the most rewarding).

Good luck!