Step 1: Tame a Giga, preferably a male with 30 points pre tame in melee (high 20s will do though).

Step 2: Find a level 5 female, tame it, punch it till its taming effectiveness is +0 levels (this is important)

Step 3: Clone the lvl 5 until you have at least 100 females (200 is recommended)

Step 4: Breed away, only focus on melee mutations, any other stat muts per breed discard the giga (melee is the only stat worthwhile on a giga)