For this achievement you have to perform 30 Advancing Guards. This can be done in offline mode in the arcade or in online mode during player or ranked matches.
I would go into Arcade Mode and select normal operation mode or simple mode, either works, your going to need the basic attack buttons a,x,and y. Choose any three characters your comfortable with. In the setting after picking your characters choose these settings
difficulty : very easy
time : unlimited
damage : lowest
fight request: off
Now to perform an advance guard your want to be blocking which is holding back on the d-pad or joy stick and then when a character hits you you will see your shield come up over your character, when the shield comes up your going to press any two of the attack buttons a,x, or y at the same time. This will active the advance guard, you will see a big purple shield come up and the words advanced guard.
What worked for me was backing all the way into the end of either side of the screen and waiting for the enemy come attack me and then when my shield blocked I pressed x and y. Then when my character got low on help i swap them out using rb or lb. I hope this helps!