Geralt is a murderous savage swordsman with actual supernatural powers including telekinesis and who knows what else. He's about 6'5" and built like a brick shithouse.
Brad Pitt's Achilles is a mortal man who sulks in a tent and has one move that utterly relies on his opponents not knowing he's going to do a pointlessly showy off target jump that could just as easily end with him self-kebabing onto even an amateur's spear.
Even in the movie he seems to rely mostly on people being scared of his rep. Eventually he is defeated by a credibly accused rapist, kingdom toppling gadabout and warmongering kidnapper -- who sensibly stands well back and shoots him with arrows. This after his incontinent dad refuses his very sensible request to burn the giant obvious trap. Now, it's been a minute since I read the Aenid, but I don't believe they find the Horse beside a bunch of plague-ridden corpses - but uh - even ignoring the fact that it looks big enough to be full of murderous Greeks, it's almost certainly festooned with PLAGUE. WHY YOU TAKING IT INTO THE CITY FOR MARDI GRAS PARADE? AND WHY DO YOU WANT THE GREEKS TO MAKE IT SAFELY HOME BY GIVING POSEIDON THEIR TRIBUTE!!!!???? In the Homer epic it's Athena's tribute anyway and a priest guesses it's a trick, even saying, "YO this might be full of Greek murderers don't take it into the city." But Poseidon has him and his kids strangled by sea serpents because Poseidon is Team Greece. Even Helen of Troy, who is either Stockholm Syndromed or just an asshole, tells everyone the horse is probably full of Greeks, but everyone is all, "nah it's fine, let's not even check." Also, Odysseus is in the horse, not Achilles.
THAT MOVIE SUCKED. Like, a historical retelling of the Iliad - cool. Mixing it with the Aenid and changing the story for largely pointless reasons? Not cool. Mind you, there's a slight possibility - like King Arthur, that Achilles is based on a real person, or amalgam of persons.
If it were the Achilles of the Iliad, invulnerable and son of a god, there would probably be a real match - but even then - not knowing anything about Witcher powers - Geralt could probably scan him with magic or whatever to see his mum's handprint on his heel or the Styx juice all over his body except for there.
