(This is what whiffs most scout kills too, you miss the head and hit the shoulder for a non-lethal attack). Make sure you don't have 2 people hitting the back, this is what messes up most takedowns. Missing the head will fail the kill and it is the hardest spot to hit. Whoever is hitting their head must hit the head and not the neck or shoulder, that will miss the damage bonus and you wont get the kill (since someone else hit the back/neck damage bonus). The 4th person should stagger their swing to be a second or so later and only swing if the first attack fails. A confident team will only use 3 people to kill a giant, occasionally only 2 people. Attacking the same spot a second time will deal less damage than attacking a new spot, dont plan for redundant strikes in case someone misses, that is what the 4th person should be preparing for. Make sure everyone is standing and not crouching before you hit. Most giant takedowns are done with 3 people. With 2 bonus damage hits, it is possible to 3 man stealth a giant.
Once a part has been gibbed (one full swing will gib any part it hits), it will no longer take full damage and the damage bonus will not apply. The head and back hits deal most of the damage, and the body hit is there to finish them off. Then one person to hit the body for no damage bonus. One person to hit the back for the back damage bonus. You want one person to hit the head for the head damage bonus. Sounds like people are hitting the same spot more than once.