Make sure Quetzacoatl (or however is spelt! ) know SumMag +10% 20% and 30% (this takes a bit of levelling up before the SeeD test), and that your other two characters have a lot of Thunders, and they have Magic junctioned for use. Encounter the Spider monster on the bridge after fleeing from the first battle and avoiding the next two by quickly running left, and then walking down the hill. Just hit the spider with Quetzacoatl and Thunder, and eventually it will explode of its own accord.

You will get the 100 bonus points which, depending on other factors (i.e. your other ratings), you should get SeeD Level 8, 9, or even 10 in the first instance.