This one is what I consider a gimmick mission. You are not given full freedom and it has some mechanics that you need to follow to clear it. As far as the difficulty goes, it’s quite easy; however, the final fight can get a bit challenging if you are not careful. If you are having trouble with clearing the mission on Titan difficulty, here is a detailed Age Of Mythology: Retold mission 6 campaign guide.

TLDR

• Train villagers only; the starting troops are enough to defend.
• Put villagers on food as well as wood
• Raids come from top or bottom; fight them off and kill runners with bolt if needed.
• For the second half, use Meteor on the far left fortress and the right fortress.
• Micromanage your troops and fight in skirmishes instead of pulling the entire army.
• Colossi should always go for buildings and never troops.

AOM Retold Mission 6 Titan Video Guide: 

Here is a video guide if reading isn’t something you fancy: 

Objectives: 

Build the Trojan Horse.

• Accumulate 1000 wood to build the Trojan Horse.
• Prevent Trojan scouts from giving away your location (optional).
• Build the Trojan Horse.

Destroy Troy.

• Sneak your heroes through Troy toward the Trojan Gate and find a way to destroy it.
• Kill the Cyclops guard to steal the Helepolis siege towers (optional).
• Use the Helepolis siege towers to destroy the Trojan Gate (optional).
• Defeat Troy by destroying the three Fortress within its walls.

Step-by-step Guide

General Tips:

• This is a gimmick mission that is quite easy. That said, pulling too much at the end of the fight might cause a failure. 
• The start of the mission requires you to collect 1000 wood. Instead of sending your villagers straight to the woodline, collect food to produce more villagers, which will make the mission faster. 

• Use your three god powers whenever needed to deal with the raids. Use the Bolt on anyone that gets too far away from your troops.
• For the second half, although you get a massive army to control, it won’t take long for it to fall. Fighting the enemy in waves is important instead of charging headfirst.
• Use Meteor god power on the far left Fortress and on the right to make the fight as easy as possible. 
• Use other god powers as needed, and remember to use Colossi as siege units and not frontline infantry fighters. 

AOM:Retold Mission 6 Campaign Guide Titan Difficulty | In-Depth Tips: 

First Half

• Put your starting villagers on food instead of wood. Autoqueue villagers and command them on wood. 

• Taking the TC as the middle, you will be attacked from top and bottom with small raids of cavalry. They are not enough to do any damage but killing them results in no extra troops being sent after you. 
• The starting army you get is enough to handle raids for the entire duration of the first half.
• Once you get enough wood, start the construction by sending all but one villager to create the Trojan horse. Leave the solo villager on food.

Second Half

• I suggest saving at this part, as you might end up losing your army.
• Once you do the stealth part of the game and break down the wall, grab your entire army and wait. 

• Use Meteors on the far left Fortress and on the right; sometimes the far left Fortress gets destroyed because of it. If it doesn’t, we can send the Colossi to deal with it. 

• Do not fight the whole of Troy instantly, as it will result in a quick failure. Bait out parts of the army and deal with them one by one. Remember to use the Colossi as siege units and don’t send them to fight troops. 
• Once you deal with most of the army defending a Fortress, send the Colossi to destroy it while ignoring the remaining troops.

• The right side has a slightly larger army defending it; my suggestion is to draw them out by sending any remaining infantry and siege towers you have and then throw the Colossi at the Fortress. 
• Don’t forget to use the God Powers when needed. Blot is good for dealing with Hydra, while Bronze and Restoration can be used when you are fighting a relatively large army or pushing the Colossi to destroy the Fortress. 

Need help with other missions as well? You can find a detailed guide of the entire campaign here.