Oh and to even unlock "I am Ugh-Zan" difficulty you have to play through the whole game on "mental" difficutly and for that one you first have to play through the game on "serious" difficulty...
or instead just type in the following command in the console to instantly unlock all difficulties:
/sam_bUghZanActivated=1

"I am Ugh-Zan" is actually pretty easy. As you can actually get this in coop split screen even though the description says Singleplayer. And in coop just use a buch of modificatiors like infinite ammo, 10k hp, high walking speed, high fire rate. I played the grand cathedral level for this which was pretty easy.

And follow this guide
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1264764384
to get the Christmas 2014 achievement way easier

Sadly leaderboards are non functional broken abandoned. Some new survival maps scores simply do not register and are bugged. Any existing score if you beat better does not update.

The coop trick to only play through the last level of a campaign workes, however for TSE you will not get the achievement if you quickly go into the cathedral after killing the boss. You actually have to wait quite a few seconds after killing him, then the achiev pops up.

Thanks for the guide! However I think I found a better way to farm "Stop jumping!".
In the second level of Bright Island - Ruins of the Abyss there will be like 5 marsh hopper spawners at the end in the arena fight.They will infinitly spawn hoppers for you to kill

A friend and I recently finished TSE in coop where we played the full campaign but reloaded saves over the course of a few days. We did get the achievement to pop. But it seems people's experiences vary here.

I am, Ugh Zhan use Grand cathedral. First spawn area Fight duck behind right side minaret kill soldiers. Then step out finish rest. Ammo use very limited big enemies are 2x-4x stronger. Then hide in secret shield corner using 3rd person view to see enemies. Lure kleers 1 by 1 to corner when they pass your side reduce numbers then step out. This corner is best to run back to and hide during every big wave of mechs. if you hide long enough big mechs teleport on other side of the yards giving you breather. in final meteor yard there is one corner near the final fight door with to rocket jump inside check where. Else you need wait near big walls in meteor yard and wait lot of as meteors hit really hard, thin out enemies from distance run in when safe along the main path to grab ammo.

At the time I didn't use any strategies to do it, just winged it. Will update the guide with your useful tips.

"Good luck with that!"
Thanks, very guide-y.

Here are some useful tips when you're doing the 2014 christmas challenge:
1. Try the challenge in 3rd person view - that helped me out a ton with FOV. Seeing if you're about to be surrounded much easier.
2. Within the first 3 minutes run clockwise. The suicide bombers spawn on your 2 o' clock from the start. From then to the end, go counter-clockwise, since their spawn points switch (or just spawn from all sides, I couldn't really see).
3. At around 3:15 - 3:30 grab the serious damage and mega health, that's your game changer.
4. Whenever a fiend demon appears (tossing the orange fireballs) they should be your priority. The fireballs can knock you back into a mob and you can easily get crushed.
5. There are quite a few health pickups and 2 armor spawns. Replenish those as often as needed. Be sure to kite the mobs behind you to not get surrounded too quickly.