Luckily, there's a good example out there, so I used it and put it in my spawn.json like this

[
    {
        "hostile": true,
        "when": "finalize",
        "result": "allow",
        "healthmultiply": 10
    }
]

This worked well, but there was one problem
Mobs holding weapons like skeletons spawned without holding weapons.
Is there a workaround?

Resolved
Changing allow to default worked
I would recommend changing the Spawn: beefed up mobs part of the example from allow to default