So for modders there's 2 new globals. Enemy Hp and weapon properties. In weapon properties you can change stuff like bullet damage, ammo and that's basically it. And Enemy HP does literally what it says.
You do TF2_EnemyHP[MT_DETON] = 666 AND now detons need to take a lot of damage with a shotgun.
Btw only for the shotgun.
And that's it.