my 1 = 168 mutation rex 2= 84 mutation rex 2= 42 mutation rex total 5 rex defeated overseeer in 10 or little more minutes. it took almost 200 rex to get 168 ranodom mutation mutation. all start from tamin wild 900 level female and 850 male to 1230 level and health 200k damage 5k and more . buy god console or take ark pass to set difficulty level 30 to spawn 900 plus dinos and use extraordinary kibble to tame the rex it fill lots of persentage. if you get success then start mating i increase the taming speed so it took less time but if you do normally it took lots of time and patience. so good luck.