Said group partakes in this competition all the time, they literally take people from all around the world to test their combat skills to see who's the best and who can lead them.
So your Character A does well against the group in combat?
Yes he's considered one of the legendary heroes of the series.
For the context of the legendary hero, the rival character who's trained under him can go on a one man army against a bunch of high ranking soldiers, including those under similar training and mindsets as the group.
Training under someone still doesn't imply that they are as skilled as their mentor. Even if Legendary Hero LH > Skilled Group SG and Rival (Trained by LH) > soldiers (trained by SG) doesnt mean that Rival > SG. That is, viewing this point in a vacuum.
You said that A already fought and performed really well against SG in that tournament, so A would be <= SG. If the implication is that A trained under LH is > Regular A then it would be okay to assume that A trained under LH is at least =, likely > SG.
Now to list some counterarguments ala Devils advocate because I know jack about the verse youre talking about, and if you can answer them based on your knowledge then A it at least =, likely > to SG:
1. Is regular A <= SG based on his performance in the competition? Just doing well in it doesnt necessarily mean he is compareable in skill. In a fight, many factors take place, skill is only one of them. Considered false if A fought members of SG and did well based on skill (Either by being stated or by being reasonably inferred from the fights)
2. Even if LH is > SG, that doesnt mean that anyone training under LH becomes > SG, because training doesn't guarantee that you become compareable to the one you train under. Considerd false if training is implied to make characters compareable to the one they train under.
3. Even if Rival trained under LH = to LH and Soldiers trained under SG = SG and thus Rival trained under LH is > SG, doesn't mean that A is also > SG, as training isn't some static equippable that raises anyone to the same level or even raises everyone by the same amount. Different grow rates and limits exist for any person afterall. Considered false if A = Rival (Both trained by LH)