Alphonse vs. Barry the Chopper
SPOILER!
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Episodes 20–21 · Laboratory 5 · Result: Barry attacks Al's identity instead of only his armor
Barry is another soul bound to armor, but unlike Al he is delighted by what he has become and has no interest in behaving like the human being he once was.
The Breakdown
Barry finds Al's weakest point and attacks it verbally: if a soul can be attached to armor, perhaps Ed simply fabricated Al's memories as well. The fight becomes harder the more Al doubts himself.
WHY IT MATTERS:
Barry nearly wins by making Al question whether there is an Alphonse Elric to defend. It is one of the few fights in the series where the decisive weapon is an idea.
Edward & Alphonse vs. Scar
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Episode 14 · Central · Result: the brothers discover how badly outmatched they are
Scar attacks the Elrics as State Alchemists and tears through Edward's automail before either brother understands the full mechanism of his right arm.
The Breakdown
Scar does not need a flashy exchange of techniques. He analyzes the structure he touches and stops the alchemical process at destruction. The fight works because Ed's normal improvisation suddenly has no time to become clever: contact itself can end the battle.
WHY IT MATTERS:
This is the moment Scar stops being a rumor attached to dead State Alchemists and becomes Edward's personal evidence that the title he fought to earn has made him somebody else's legitimate target.
Edward vs. the Slicer Brothers
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Episodes 19–20 · Laboratory 5 · Result: Ed survives two souls in one armor
Edward enters Lab 5 expecting guards and finds an alchemical variation of his own brother instead: two convicted killers bound into a single suit of armor.
The Breakdown
The fight becomes a problem of blood seals rather than conventional anatomy. Destroying the armor is not enough, and discovering two separate souls forces Ed to rethink how reproducible Al's condition may be.
WHY IT MATTERS:
The battle turns soul binding from the desperate miracle Ed performed once into technology other people have learned to weaponize.
Edward vs. Sloth
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Episodes 46–47 · Result: Ed destroys the Homunculus created from Trisha
Sloth can turn her body into water and repeatedly uses Trisha's face against the brothers emotionally as much as physically.
Edward ultimately uses the remains connected to their mother to weaken Sloth and forces himself to finish the fight.
The Breakdown
The mechanics and the character problem are the same thing. The object capable of weakening Sloth is proof that Trisha died and that the being standing in front of Edward is somebody else.
WHY IT MATTERS:
Winning requires Ed to stop treating resemblance as resurrection. He cannot undo the original human transmutation until he accepts what it actually created.