Tales of Vesperia

Have only Rita attack with magic during the fight (best if she is in your party when you go to the colosseum). When Zagi absorbs the magic for the third time, his blastia will break. At that point go all out on him and end the fight.

*Spoilers*

"Third battle VS. Zagi. Let him absorb too much magic, destroying his bohdi blastia."

When signing up at the coliseum, have Rita in your party.

Go to the menu and go to Strategy. You can do this during battle if you like.

When you fight Zagi in the coliseum, have your DEFEND strategy like this for Rita (you can leave everything default except Rita):

Rita
Same Enemy
Full Attack
Full Artes
Artes Only
Keep Distance
Watch HP
Use Lv.2 Over Limit
VS Boss

Use this strategy (hold LB in the menu during the fight). Do alot of blocking with Yuri until Zagi absorbs enough of Rita's magic to where he falls over and you get the confirmation "Great".

Then, switch back to a different strategy and beat Zagi again.