It's hard to get it with bots because they can kill you while you are running away.
Other method is dealing damage to yourself with the granade or the bazooka but you can get kill or don't drop the health below 20 and you must wait till the game gives you more ammo to use them again.

You can get it alone in a particular map and i'll tell you how:

Create a match on the map "Picnic Panic" without bots and for only invited players (so you'll be alone with the NPC enemies)


On this map you can find 3 differents insects that always hit you the same:
1) Ant: deals 15 of damage so you need 6 hits of them (100 - 90 = 10 hp)

2) Spider: deals 30 of damage so you need 3 hits of them (100 - 90 = 10 hp)

3) Beatle: deals 50 of damage so it will kill you at the 2nd hit!

I recomend the spider because with the ant it's to slow and the beatle is risky.
Tip: while you are regenerating go to a spider when you are near 50 hp so the hit will let your health below 20 (example: 45 - 30 = 15 hp)

Over time, the insects begin to overlap each other so it makes it difficult to know the exact damage they are gonna deal: start killing them till you leave the ones you want.