Step 1: Buy a magic
Obliterator Bow from the market with as high "increased physical damage %" roll as possible. The T1 ones are expensive, so I generally go for T2. Suffix doesn't matter.
Step 2: Use a "Greater Essence of Abrasion" to add physical flat damage to your weapon and make it rare.
Step 3: Use an "Omen of Sinistral Necromancy" in combination with a "Omen of the Liege" and desecrate your weapon. This will guarantee a prefix that is of the Amanamu type, making sure you get the Companion % damage once you unveil it. Don't unveil yet. Your prefixes are now filled.
Step 4: Fill out the suffixes with 2x Greater Exalted Orbs to guarantee that your essences will remove suffixes.
Step 5: Use a Perfect Essence of Battle to add +4 Attack Skills. You can also do the step 6 first, if a perfect essence of haste is cheaper than battle.
Step 6: Use a Perfect Essence of Haste. If it hits any suffix that is not your +4 Attack Skills, then you are done. If it hits your +4 Attack skills, then you will need to use a Perfect Essence of Battle again and repeat this step until you have both +4 Attack Skills and Onslaught.
Step 7: (OPTIONAL): Activate an omen of Abyssal Echoes. Only do this if you want to make sure that you get a high roll on your companion damage.
Step 7: Unveil and grab Companion Damage.
Step 8: Enjoy