How do you see all 150 Pokemon?

After the elite four, you will have seen most of the Pokemon if you battle every trainer you see. Here are some Pokemon hard to find in sinnoh...

Riolu:Hatch egg from Riley or go to the Oreburgh gate, after you get the bicycle from Eterna City, and go up. you will need surf and strength to do this so at least after the canalave gym you should use this plan. Anyway, go on the lake and battle the vetaran on the other side, to the west. He will have a Riolu.

Lumineon: On the route to the victory road, one of the girls will have a lumineon.

Combee: On the route to the west of Hearthome City, there will be an aroma lady who is the only trainer who will have a combee.

Dialga(Pearl)/Palkia(Diamond): Talk to Cynthias grandma after beating the champion in the big house to the north.She will show you the opposite of your game cover.

That's it from me!!! If you have more Pokemon answer and that will help!