Sparing Sans before he asks to be spared

Here's the ideal item list you should have:

• Butterscotch Pie
• Mettaton Steak
• Instant Noodles (You can only take one IIRC)
• Three Snowman Pieces
• Two Legendary Heroes

Other notes:

The ATK of your weapon doesn't matter at all.

When Sans slams you down into the bones, hold the direction opposite to the one he slammed you. You'll jump out of the way. It's sort of like him flipping the gravity of your SOUL. Note that you'll need quick reactions to actually dodge this as the attack gets faster later on.

Learn to be quick in menus. If you've gotten far enough into the fight, he'll start attacking your SOUL cursor in the menus!

Using an item will not advance his dialogue, and thus, won't advance the fight. Attacking him is what triggers the next line, so use as little items as you need to. This is why the Butterscotch Pie is crucial - it maxes out your health in a single turn.

Also, just so you know, on the genocide route, the instant noodles work the same as the Butterscotch Pie. -Tastes better dry

Unmarked spoilers ahead (since spoiler warnings are included in the question).

It seems like, reading this sentence, that Sans already fought and beat Flowey many times in the past. But, during the whole game, it doesn't seems like Sans has ever met Flowey at all.

Both these statements are true. Flowey has the ability to Save and Load.

After acquiring this ability, Flowey first tried to help the denizens of the Underground but got bored and began messing with and killing them. The resets he mentioned were caused by him being discovered by and defeated by Sans.

Sans is aware something keeps reseting the timeline but, unlike Flowey, cannot Save and does not keep memories of other timelines. He doesn't know of Flowey's identity (or perhaps even his existence) in any timeline in-game.