You have two options. Talk to your DM, or Find a new DM.
End of the day, you can't force a person to change and No D&D is better than Bad D&D. So if your DM cannot be persuaded to change their DMing ways, your only other option is to find a new DM.
So, your only non-quitting/non-firing-the-DM approach is to get your DM, preferably alone, somewhere away from the game table and not during game-time—and have a chat. I strongly recommend trying to be non-confrontational—approach this as "we're trying to fix this" not "you are a bad person and you should feel bad." I say this even if you think your DM is being a bad person, simply because accusing someone of that immediately puts them on the defensive. Here are some recommended talking points:
D&D is supposed to be Fun. We are not having fun.
D&D is a social game in which everyone—the players and the DM—are supposed to have fun. The standing rule within D&D is simply that: Have Fun. If memory serves, the DMG even explicitly says that if a rule of the game is getting in the way of you guys having a good time—ignore it!
Express to your DM that you guys are not actually having fun. You put a lot of work into your characters and you want them to have fun adventures. And the fact that he kills your characters and then criticizes you for not being "good enough" makes you not want to keep playing in his games.
Keep in mind that some players like high difficulty games—games where the Players have to be very optimized and very skilled in order to survive and win. Acknowledge this. Then tell him that you (and your friends) aren't actually interested in that sort of game.
D&D is not (usually) adversarial
Maybe he doesn't understand the point of the DM. Maybe he doesn't understand that the DM is not supposed to be the enemy of the players—but the one who gives them challenges along the road of telling a story. It sounds like you've tried to communicate this—but it's a very important point.
The DM should only be seriously trying to kill the PCs if that's the sort of game the players want.
All else fails
Now, you bring the rest of the party in—make sure you're all on the same page, and talk to him about this as a group.
Inform him that as you guys are not having fun—you'd rather not play D&D with him as a DM. D&D is not about players being "Highly skilled, hyper-competent masters of the game," it's about friends getting together to have fun. As none of you are actually having fun with D&D as he is running it, there's not much point in continuing to play. So, if he's going to stick to his "Git Gud" guns as a DM (reminder: you're still trying to be non-inflammatory here--so don't use those words)—then you're all simply not interested in being players under him. He may be able to find some more hardcore players to DM for—but you're not really interested in playing a game the way he runs it.
Tell him that if he's willing to try running the sort of game you all want to play then you'd be happy to give it another go (if, in fact, you are). Otherwise—he's welcome to continue playing with the group as a player... but you'd rather not have him as DM.
I mean... end of the day, you guys showed up expecting to play Skyrim, and he's throwing Dark Souls at you. You're not playing the game you wanted to play.