I'm currently level 118, and I'm stuck in the second fight with Mephy since Momo keeps diying. When I went on youtube, the videos I saw had the player chara just solo Mephy without Momo. At my level, I use bows btw, I only got this far just because of my level 80 Hound Dog and help from higher leveled players. At this point though, I barely do damage to the bosses in the game, or even most of the monsters (looking at your gargoyles), and I've been spamming Exorcist Shot and Amaya's. I keep ramping up my weapons, but they don't do much for me, I still barely do much.

How do you end game players actually reach that end game status? Is it good weapons? Some magatama? Fighting something like Mephy with shit damage for 60 minutes or more?

If it's the story second Mephisto fight, you can just spam Possession with Momo.

Yes, it's good magatamas, weapons, and a lot of skill and careful observation.

For Meph's second fight, as Miyano said, spam possession. Attack Meph from the sides, and do hit and run tactics. When he falls to the ground and starts smoking, run to the other side of the map as fast as you can. You'll be able to avoid his ground lightning attack that way.

It's going to take a while, but memorize his attack patterns and only attack as Asura. Run when you can and immediately swap back to him when the vanguard meter runs out.

The game is really easy now. Even for low level players if you follow discord group and new event update.
Is your Hound Dog at 15 scrolls (2000 stamina) ?
Do you miss the last few events where you can get fusion bear/fused bear mags; wind/lightning mastery mags quite easily?
These mag will get you through the game easily.

Do you make a lancelot weapon (RevII which is a lvl 120 wp for sword/spear/odachi/twin is really easy to make)?
This is for every new players (at least in PC and ps4), it is 2018, skip the satan weapons, or serverance. Go straight to RevII first if you are still weak for Gargoyle/dragon ark.

I forget to mention that from the last update, you can buy the new 'zunko mags' from Miroku. These mags are not the best, but good enough for low level player to catch up.