Terra is a very easy boss. Basically, Ventus can use any method possible to reduce Terra's HP to zero while trying to avoid taking damage himself. An easy way to defeat Terra is to activate the Shotlock, which will deplete Terra's HP to zero in a matter of seconds.
Terra may be mostly close ranged, but he has the Sliding Dash command and uses it often if he is at a distance.
Standard Combo: A three to four hit combo that deals a good amount of damage, but can be easily parried/dodged.
Sliding Dash: Terra charges forward to close the distance as well as deal some damage.
Brutal Blast: Terra performs a spinning attack that deals a chunk of damage as well as knocking Ven away.
Quick Blitz: Terra performs a jumping attack that deals damage as well as knocking Ven backward.
Guard + Counter Hammer: Terra will enter a blocking state. If Ven attacks him during this, Terra will block the attack and immediately retaliate with Counter Hammer, dealing a good amount of damage and knocking Ven away.