Concerning 1., as blksabbath74 pointed out, even with full strength one can still lose a battle due to other circumstances that may or may not be intentional. Riku also lost to Roxas first and only won the second round because he cheated, but he did win in the end.
The point still stands though that Lexaeus, unlike Axel, actually managed to win a battle.
2. No one implied/said though that it was expected to be done by brute force or battle power. In fact, most things Axel accomplished were either by trickery or choosing the best opportunity to intervene, not battle power. Vexen was already worn out and on the ropes from his battle with Sora, not to mention that Axel's first strike came as a surprise attack. He literally only finished off an already defeated man, same goes for Zexion actually as he waited until Riku had already weakened him before striking by using the Riku Replica. Marluxia is a tricky issue because we do not really know how serious they both were, not to mention that they both did not show any of their true abilities.
As for Sora and Roxas, he retreated in both fights with Sora before it could get any serious and Roxas literally trashed him in KH 2 while Saix beat him to an inch of his life later (that is only shown in the manga and the novel) so that he was barely alive when he came to aid Sora in bewixt-and-between in KH 2 where he ultimatively sacrificed himself.
Axel is certainly an effective fighter who also knows how to survive, but he's also someone who relies more on trickery, tactics and taking of opportunites rather than actual power.
This is also furthermore reinforced when it is shown in Days that, although that he won the fight against Xion, he is on his last legs when he returns to the Castle that never was and actually feints from exhaustion, dropping Xion who is then taken away by Xemnas.
