My strategy is simular to HollabackGamings but we definitely did somethings different.
For one I played on Gravewalker difficulty which if you get hit twice by even the grunts your run is a wipe.
But yet on my second attempt on it I managed to get 500745 as my score and it took me between 1H and 1H 20Mins. This is for tips on how to complete it on Gravewalker difficulty.
Heal everytime your health is below three quarters.
On your first run collect all of Torvin's items and return them (these are incredibly useful and you get an achievement)
Try to kill everything from a distance and stealthily unless the Captains traits force you to fight head on. My strategy was flying above the Captains, stealth attacking from above then getting out of there as fast as possible but had to regularly improvise.
If there are a few captains bunched together shoot in the middle of them with a poison bomb then shoot a fire bomb, this causes Balefire which is devastating.
Put epic mercs down as outpost leaders because in the final assault common mercs die before getting inside the gates.
1. The one that allows you to see Outpost Captain's traits.
2. The one that allows you to see normal Captain's traits.
3. All four of the healing upgrades.
4. It doesn't really matter from here but I suggest the bolt or bomb upgrades.
Constantly check for captains immunities and vulnerabilities, if one is immune to arrows but vulnerable to stealth its wise to stealth attack them and if you have full might when you land, use your combat execution on them or a a vulnerable captain then spam the A button to jump over the grunts heads until you find a place to latch onto and escape, then reset and do it again until they're dead or you've captured the outpost.
I only had one bodyguard and he was legendary which I bought just before the assault, activate him once in the final room where you face the overlord as this will really help.
I didn't kill any of the Warcheifs before commencing the assault so was forced to kill all four catapult graugs (which needed to be taken out quickly) and the captive drake. Your mercs should clean up the enemy Warcheifs so just swoop in for an easy kill or taking one of their defense points.
For items make sure to check them before activating them as one halves your health which on Gravewalker is a death sentence. But make sure to constantly update your items.
When in a 1 on 1 with a captain (this applies for the overlord) use your chain to reel them in to you then when they're down ground execute them, rinse and repeat until you have enough might to do a combat execution, then continue until they're dead, this way you shouldnt take any damage and should clean them up quite quickly.
Really handy trick by Place of Power
: "When your about to die while in the kneeling position you can dashboard/quit game and it'll be like nothing happened, the map will still be blue. While I was about to die while kneeling in the overlord room I quit the game immediately and it only took me back to the start of the siege... the map was still conquered and I got my gold rating."
Goodluck guys and I hope this helps.
Edit: added photo as evidence of my score.http://s1380.photobucket.com/user/Halo0006/media/20180514_22...
Edit 2: was asked for extra tips so added them in here too.
Edit 3: I was given a really handy trick by Place of Power
to add in here.