Soon wasn't really "on the battlefield," though... he was holed up in the throne room.
I assume that Roy would have challenged Eugene's statement - which was essentially that none of Roy's party members were as high-leveled as he was - if he knew it to be untrue, and I assume Roy knew what level the other Order members are. It's pretty important to know that sort of thing in tactical planning, y'know, and even if we don't always know when characters level up anymore, I'm sure Roy always did, at least until he died.
Presumably Eugene knew that if his criticism of Roy in that instance was flat-out wrong, Roy could have refuted him and made him look stupid by saying "no, I'm the same level as everybody else," or even by plausibly being able to say "no, I have no reason to think that I'm the highest level character in the party and thus the highest level character on the side of Good." Thus I assume Eugene actually knew what he was talking about.