THIS ACHIEVEMENT IS MISLEADING: YOU HAVE TO COMPLETE THE CAMPAIGN AND THE ADVENTURES TO GET IT, HENCE THE PERIOD AFTER CAMPAIGN. IT SHOULD BE READ AS 2 STATEMENTS. WATCH THIS:
COMPLETE THE CAMPAIGN.
(and then) DEFEAT THE HARBINGER!
FOR THE SHORT, SHORT, VERSION SKIP TO THE BOTTOM OF THE SOLUTION WHERE IT WILL SAY.......WAIT FOR IT.....WAIT FOR IT.... THE SHORT SHORT VERSION .
FOR EVERYONE ELSE:
Start up the game.
Select "Go to Town".
Once you load into Town, go to the " War Table". It should be directly in front of you in the center of the room, which happens to be in a stone cathedral.
From the War Table press DOWN on the Right Stick (RS). DO NOT HOLD DOWN, JUST PRESS IT BRIEFLY.
From the "Mission Menu", as I call it, use the RB or LB buttons to scroll over until you get to ADVENTURES.
***FROM HERE- IF YOU ARE FAMILIAR WITH THE GAME MECHANICS SKIP TO THE LINE: "HARD DIFFICULTY IS NOT REQUIRED FOR THE ACHIEVEMENT".***
If this is locked out then you haven't met the perquisites. Read the maps to see what you need to do to unlock it. I suggest completing the campaign on both NORMAL and HARD difficulties then jumping into Chaos I on Incursions or Expeditions and leveling up some gear first. If you are running with at least Chaos II or III gear, you should have no problem doing this and you are ready, but if not then I SUGGEST YOU REPLAY "GATES OF DRAGONFALL" SOLO ON EXPEDITIONS CHAOS I OVER AND OVER UNTIL YOU CAN SOLO IT COMPLETELY AND THEN DO WHAT YOU CAN ON CHAOS II.
When trying to solo use a Private game. When making CHAOS II and up runs make it public, people will usually jump in at random and can be helpful. However, use the RS + Down on D-Pad to inspect their defenses. If they are significantly weaker than what yours are they should either be upgrading what you place or dropping mana (RB + Left on D-Pad). Due to build limits this is important.
Alternatively, If YOU are the weak character do likewise and either let the more powerful characters build and you upgrade or drop mana to them. If you look for a CHAOS III match at the table and jump into one be weary this.
*REMINDER*
EACH CHAOS TIER IS LOCKED AND LINKED TO A SPECIFIC GEAR RANK REQUIREMENT. ONLY 1 CHARACTER IN YOUR "PARTY" HAS TO BE AT THIS LEVEL TO ACCESS IT. Many players will go all in on 1 specific character for this reason.
Once your Characters are around 1000+ Gear Level (It's the little cog in the corner of your character card in the Inventory screen) you should be about ready to solo it on normal.
HARD DIFFICULTY IS NOT REQUIRED FOR THE ACHIEVEMENT.
Each map has a prerequisite. You don't have to play every map on the First 2 sections only unlock them all (i.e. you don't have to play the last map in the first 2 sections as the first map should be unlocked in all 3 sections already, at least mine were. If all yours are locked then I suppose you would have to play each one.)
There are 3 sections to Adventures.
The Kobold Bling King
3 MAPS (The Forgotten Ruins, Nimbus Reach, Little-Horn Valley)
Mal'donis the Terrible
2 MAPS (The Dead Road, Temple of the Necrotic)
The Harbinger
3MAPS (Ramparts Siege, Assault on Throne Room, Harbinger's Warship)
Unlike the other maps, Harbinger's Warship is an actual BOSS FIGHT.
HARBINGER'S WARSHIP
There are only 3 points of entry to the core. Set up sufficient defenses as to your liking. At one point, a Warship will get called in and will bombard the core from above. I had 2 anti-air towers (Monk ability) flanking my core but I didn't notice if they were that effective or not.
Take out the cannons that are facing your core ASAP. They appear when the round starts and the Harbinger will use them to shoot your core doing massive damage. After this, immediately attack the Harbinger directly. Stay clear of the back of the deck of his ship (back of the map) as he will try to lure you there and rip away the back wall revealing a large hole that will try to vacuum you in. You can not place traps in this part of the ship, this will be a straight up melee.
If you do get sucked into the ships hold (THE VACUUM HOLE) you will find yourself in a poison filled toxic room full of what looks like green acid, toxic clouds, and some cages and pipes.
IF YOU GET SUCKED IN:
IMMEDIATELY LOOK UP FOR A SMALL PIPE WITH A BLACK HOLE IN THE CENTER. IT SHOULD BE IN A CORNER OF THE ROOM.
USE THE MONK TO TRY TO DOUBLE JUMP UP TO AND INTO IT QUICKLY AND YOU WILL FIND YOURSELF BACK ON THE HARBINGERS SHIP DECK. CONTINUE THE FIGHT.
***You have 90 seconds between rounds to do repairs/build. If your defenses cannot keep the hordes away from the core during the waves without your assistance, you may not be ready to solo this yet. ***
Eventually, (Usually in the 4th wave) the Harbinger will Jump onto your deck and attack the Core directly, TAKE HIM OUT!
Watch the cinematic ending.
The achievement should pop sometime during the cinematic.
THE SHORT SHORT VERSION
FROM THE MAIN MENU :
GO TO TOWN
ACCESS THE WAR TABLE
RB/LB TO ADVENTURES
COMPLETE THE HARBINGER'S WARSHIP MISSION
ACHIEVEMENT IS YOURS!