My advice for anyone wanting to mod in higher difficulty, here's the advice of someone who's done a fair bit of "element zero" / "diamond" difficulty modding...
1. Don't make enemies have loads of health... this is just tedious.
2. Use variation in your waves, use an enemy cap of 50+ and hard cap the individual units, so if you want you have capacity to make a wave with many husks or abominations you can.
3. Up the game speed. Making people have to react and play faster with enemies which fight and move faster is much more fun than giving an atlas 6 million health.
4. "Boss" units can be really fun, but limit them. Sure, have a possessed praetorian with 500,000 armour on wave 10, but have only 1. Possessed collector captains, scions or troopers can also be fun boss units.
5. If you are making boss units, heavily reduce the stagger / flinch %s, meaning you can't flinch lock scions, etc.
6. Try messing with enemy weapons. Giving cannibals highly inaccurate miniguns adds an interesting dynamic. I also modified nemesis weapons, so they fired like raptors, only dealt tactical cloak levels of damage. If you got struck out of cover by a burst, you died.
7. Adjust the "interplocspeed" stat for enemies baseline. This is the stat which controls 99% of the enemies ability to target you. Reducing this number will drastically reduce the amount of time it take enemies to "snap" onto you and start gunning you down. This is also frame rate dependent, so depending on what FPS your host is running, you might need to adjust accordingly. Generally speaking, 60 fps hosts should reduce it by 50-75% depending on how difficult you want the enemies. 120 or higher fps hosts should be very cautious about reducing it significantly, as this rapidly makes the game impossible when every enemy has pin-point accuracy and tracking from the second they catch a glimpse of you.
8. Rapid fire ravagers. You know you want them.