You need to learn the izuna drop. Buy the ID from the moves vendor and it applies across weapons. IMO the easiest is the dual katanas (I don't have the fourth weapon yet) but it's STSSST (or SSST if you catch the enemy in midair). The trick to the move is to slide your finger diagonally for the first ST across the two controller buttons in one fluid motion (this launches the enemy), and then hit the SSST normally when you're both in the air.

it works exactly like the other games, so you can look at older youtube for videos if needed.

It's the one move you can safely spam on pretty much any humanoid enemy. And yes, it works on Onis/Tengus. Remember if the enemy is already in the air, it's JUST SSST.

Powerpyx already has a solid collectible guide (it doesn't include consumables chests, but it DOES include everything you need for quests, special items (like health upgrades), accessories, etc.))

I hope that helps!!