For example, if it's a full house game the enemy waves should have more HP than let say, 2 player present in the game.
I have came up with a set of numbers that accurately scales with the number of players present at any given time.
This implementation requires that you check for how many players are present before every round, and set the enemy wave hit points to the following:
PLAYERS IN GAME / ENEMY HP
1 - 45%
2 - 55%
3 - 66%
4 - 76%
5 - 85%
6 - 93%
7 - 100%
It took me a while to come up with these numbers, and I can confirm it works well after countless testing with Turret Defense Remastered.
If you would like to offer multiple difficulty options, simply use the above scale as HARD or NORMAL difficulty and for easier difficulty, reduce all HP by 5-10%.
Hope this helps if you are making defense maps or something like DBZ Saga where you have to fight enemies in a co-op setting.