Identify which genes you like the least. For me, that would be:
1. Fortnite snout
2. Digging paw/claw combo
3. Heatbody anywhere heatbody shouldn't be/ Spiky tail-eater
Set your nichelings up as Adam and Eve, but make it sandbox. Despair. Remove all wanderers or give them purebread bad genes.
Rogue males should be consoled to have your favourite recessives. (Fangs/purrsnot, scorpion tail). Optimally, even throw in a pattern or eyecolour, something really sneaky. If one of your line is born with the "good" rogue male genes within 200 days, it's game over. However, rogues are immortal and females must give birth if impregnated.
Starting islands: burning savanna, whale island, first snow.
Survival chance: 5%