A. High stance attacks and every Active skill cannot be deflected so you want to build your combos around them. Fists, tonfas and Switchglaive are top tier for this. Dual Sw and Hatches are good as well because they have a skill that u can activate if your attack gets deflected. And Axe cannot be stopped. 

If you still having trouble, temper the "Break" stat(or get the starred version) on the weapon. Also learn Zero-Ki Combos (LastToLoad on YouTube has great vids), this is a tech to keep humans at zero or low ki, if you grapple they will be at full ki instantly.

 Also add yokai abilities to your combos, these attacks will skip animations of attacks to make some cool and fast combos, these cannot be deflected as well.

B. Against yokai, your main focus is to get there purple bar to zero after which you started your big combo chains. So build small combos that do good ki damage or have ability to break horns and other weakspots, Yokai abilities do tons of ki damage so use them as well.

If you got time watch LasttoLoad and PooferLlama videos on youtube. Both have guides up, Llama has his stream VoDs where he plays every weapon and does alot of combos so watch bits and pieces of them if u don't have time.