User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170327215000/@comment-31137559-20170328012308

Unless Issei fought a god in the timeskip or lost to Sona or some other named character, you have nothing to worry about. After his match with Barakiel, he won some matches before having his match against Dulio.

And getting into the top 16(I'll say this for the thousandth times) isn't dependent on how many matches you won. It depends on how many Rating points you have. Works like the Rating Games system. You beat a team that has more Rating points than you and your Rating increases exponentially. Vice versa, you lose against a team that has less Rating points than you and your score decreases exponentially. While winning does grant you Rating points and winning every match will help you, you can gain more Rating points by giving off a good performance. Using the rules to your advantage, exploiting your enemy's weakness, etc. Rias could have fought 60 matches in the preliminaries and won every single one of them while Sona could have had 44 matches in the preliminaries and won all but 7 of them. They could have the same number of Rating points. Or Sona could even have more. All depends on your opponent and how you play the game.

We have to consider whether Team God Alliance has more points than Issei's team. If they do, then Issei beating them would give him the BOOST he needs to get into the finals. And I imagine that even if Issei has more points than them, his score would go up drastically if he beats them considering it's a team of gods and god-class beings.