I'm just confused how the fight will be, it is realistic novel where the antagonist is 40+ years old who wanted to kill my main character, because he's a threat to his company (the mc is the kid who was with his father when the father got killed,.the thing is,.my main character Don't know sht because he doesn't have memories and so.his goal is to want to get rid of the threat because his goal in life is he wants to live.)
Hmmm. This is a scenario that can unfold in many ways.
If you're going for a realistic feel, keep any physical altercations (whether w/ fisticuffs or firearms) very short in terms of actual time passage. We're talking about fights lasting 10 seconds or even less; that's usually how it goes in real life - brutal, short, absolutely explosive. Because everyone is fighting for their life (actual brawls are no joke).
If you want to have a longer fight, without making it unrealistic, consider other aspects of a potential conflict, stuff where literature excels. Make the fight psychological: perhaps the protag and the antag are in a final showdown, but they enter a tense "verbal fight" first, cause a) they're human and want to avoid conflict and b) want to unbalance the opponent emotionally first to force them to make a mistake. The dialogue could be about revelations, bringing past trauma/bad decisions to light, any kind of emotional "gut punches".
Or, make the fight about misdirection and deception: have one side try to stall (either via talking, hiding, etc.) in order to gain/reach an advantage (the classic movie scene comes to mind where the bad guy reaches for a hidden weapon after "surrendering" to the hero). On the flip side, make it so that both sides are handicapped (the hero is wounded, the bad guy is infirm due to age/health issues so both are on "equal footing"), where the fight becomes slower realistically, because both opponents fight at the end of their stamina.
(iirc, in Metal Gear Solid 3, the player could defeat the final boss - which in itself is a non-standard fight scenario, a sniper duel - by waiting for him to die of old age; an extreme example, but there it is)
Finally, you can subvert expectations, and not have a fight at all, despite setting up the whole scene for one. Perhaps the protag convinces the antag to change his ways (Paragon Shepard in ME 1 against Saren comes to mind); perhaps the antag surrenders on their own because they realize how much harm their goals/pursuits have caused; perhaps the situation itself changes, and now both sides have to reason to be opposed any longer (imagine Indiana Jones or National Treasure if the artifacts in both movies turned out to be fake; why fight over them any more?).
Sorry I can't offer you more specific examples or movies/books/games to draw inspiration from. My head isn't in the right space currently, and I'm having trouble thinking at all.
For amnesiac stories (since your protag is without memories) check out movies like Paycheck, Who Am I, Memento, The Island, and other amnesia-themed stories to see if you can draw any inspirations from their endings, and then cross-pollinate it with "Big Business" stories like The Company Men, Wall Street, Lord of War (yeah, that's a business film!), The Firm, and so on.
