Allow me to explain what just happened on Jonathan's end with the Guardian. Metal conducts Hamon. Jonathan's attack "Metal Silver Overdrive", is basically a version of "Sendo Hamon Overdrive" that uses Metal. Sending a devastating Ripple through the charging arm, rendering it totaled and causing the arm to explode on itself (because the Guardian's cannon is loaded with small missiles in Battle Mode, thus the ripple wave would set them off).
If you don't get Hamon, you should have read up on the wiki article I provided on my CS. Here's the link, one more time:
http://jojo.wikia.com/wiki/Hamon and the link explaining Jonathan's Hamon skills (Hamon is Ripple in English, btw):
http://jojo.wikia.com/wiki/Jonathan_Joestar/Abilities_and_PowersIf you need visual aid for Hamon, watch the first couple of seconds of this, pay attention when he says "Metal conducts Hamon"