For this achievement you only need to beat story mode with Riptor on any difficulty. If you wish to pair this with Rivaled!
Here is my advice
40% combo damage average.To get a 40+% combo average, only throw single strikes with the

button until your opponent is on the second life bar. At this point you should have 2 full shadow bars.
With combo assist on, jump at the opponent and land with a

strike, follow this up with 2

strikes. After the 2

strikes land, immediately press

to start the shadow move. Do this twice in the same combo, then finish with

for a combo ender.

This should put you over 40% without issue. After this lands. Finish off the opponent with only single

strikes. This way your only combo will be the big one.
For the Ultra on RiptorRiptor is the 5th Opponent. With combo assist on and the opponent showing "Danger", use an

strike followed up with a

. With combo assist active, this should do the Ultra Combo everytime.