I have created this guide a few years ago, I'm not sure what still works from it.
I managed to get the new achievents rather quickly.
Only getting a headshot mid-air was a bit tricky for me (best you kick a combine and use the railgun)

rest of guide (as written a few years ago):
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This mod crashed often for me.
Problem: all the dead bodies. All the ragdolls made the game shutter and later crash.
Solution:
bind "o" "ent_remove_all corpse; r_cleardecals"

Remove the bodies every few kills to be safe.

Select gamemode "Combine Firefight" and start killing.


Spawn command:
bind "k" "npc_create npc_hunter"




Select gamemode "Antlion Assault" and start killing.


Spawn command:
bind "k" "npc_create npc_antlionguard"



Select gamemode "Zombie Survival" and start killing.


Spawn command:
bind "k" "npc_create npc_headcrab"




Select gamemode "Xen Invasion" and start killing.

It took me almost 6000 kills to get the max. level.

These should happen along the way of getting the other achievements.

This achievement is the last one I have to get & it is annoying.

My idea is:


Play Zombies, gather a lot of zombies in a room.
Quicksave
Grenadekill
reload & repeate.

You can spawn more headcrab with the console to get a bigger number.

I managed around 150 kills per grenade.

In total it took me around 30 hours to get all achievements.
But It took some hours to figure everything out. I think it can be done in less than 20 hours.