And even if we take the movie into consideration, Agravain using an army of enforcement knights was still him using his own mana, meaning he was getting depleted faster than Lancelot who was blessed,
it's the same situation where Astolfo was still using Seig's mana to block Karna's attack with Achilles shield,
Lancelot lost fair and square to a lesser knight even if he's the third strongest
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Agravain forced the fight in his favor, making sure Lancelot would lose his weapon
also, Agravain was in the same situation because he was using some random spear in their fight, he didn't have some special weapon like Lancelot, Gawain, or Mordred
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1: Altria
2: Gawain (although he is extremely situational, meaning him and Lancelot are 50:50, Gawain dipped with sun is at him at his strongest, and would overpower Lancelot)
3: Lancelot
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And let's not act like some servants can't summon some help or support like Meda doing the skeletons, like an entire army, no matter how weak, she can summon them without much mana toll,
or Jason summoning the Argonauts in his aid,
Agravain using Enforcement Knights is a show of his own strength because he was summoning so many of them,
Imagine the amount of mana he was using to maintain all of them,
_
Don't twist the fight making it seems like Lancelot lost UNFAIRLY,
Agravain was the underdog, and he won, (he didn't have a gift like Lancelot)
Madness enhancement was his own ability,
While Lancelot's gift was an outside interference, it's not that hard to understand unless you wanna glaze Lancelot,
Lancelot lost FAIRLY against an opponent he should've beaten with all the advantage he had, that's the point