As stated by Dead0fN1ght, when you complete the Tower of Ishal, you'll get this achievement. While there's not much to say here (just like the last few DA:O solutions I've written), there is a boss at the end of this stage and I would like to give some tips (as a mage, but you can apply these to whatever class you're playing) for the boss, especially for those playing on the Nightmare difficulty setting.
Boss spoilers ahead! (nothing major will be spoiled, just tactics for this battle).
You can leave the default settings for your characters if you want to tank and spank him, or you can adjust your tactics to go full on melee and burn him down (my preferred method). If you want to tank and spank, be prepared for a long battle. Alistair and one of the Tower Guards are tanks by default and they do less damage than if you set them up for DPS (damage per second). Being that there is one mob here, you do not need two tanks and burning him down with no tanks is the best option IMO.
One more thing before I start. If any of your characters other than yourself or Alistair are injured, don't bother using an Injury Kit on them UNLESS they have an injury that effects their damage output (such as an eye injury which I forgot the name of). If they're just bleeding and have a nature resistance penalty and a constitution penalty, don't bother with them. Injury kits are too valuable to waste on characters you're not going to be using through out the game... at least at this point where you're playing the hunter-gatherer trying to scrounge a gold or two.
On the other hand, if you or Alistair is injured, use Injury Kits.
If you decide to go all DPS, then equip Alistair with your best two-handed weapon. Set up his AI as follows:
1. Self: Health < 50% >>> Use: Lesser Health Poultice
2. Doesn't matter
3. Doesn't matter
If you have the Threaten skill, make sure it's active. If not, don't worry about it as it's not vital for this fight. All this ability does is processes more hate with each melee attack which causes the ogre to attack Alistair more. Since Alistair is healing himself at 50%, having him being singled out is a good thing.
Set up for Tower Guard:
Switch to his secondary weapon (crossbow) and equip Fire Bolts if you have them.
-Just leave them alone. Once you change your presets to Archer, it will give you the skills you need.
Set up for Soldier:
Give him a two-handed weapon if you have another one. If not, use your best one-hander (a sword preferably as they seem to do more damage than blunt weapons like maces plus the critical chances are higher).
1. Self: health < 25% >>> Use: Lesser health Poultice
2. Doesn't matter
3. Doesn't matter
This gives you all the DPS you need to burn down Ogre quickly, plus it keeps one of the weak Tower guards away from harm for the most part. It also gives Alistair the ability to keep himself healthy and if the Tower Soldier takes too much damage, he'll heal himself.
It has 4 special attacks that I noticed and I fought him a few times to tweak a strat. I don't know the actual names of his special abilities, so I'll just describe them by how they appear in the game:
1. Grab - The Ogre grabs a character and squeezes them. Health will diminish as he punches you while you're grabbed. There are a couple of tactics here depending on how you have your character's AI set up. The method I prefer is likely not the usual way, but my philosophy is to get in and get out - don't give things a chance to go wrong.
I am going to assume you do not have Winter's Grasp. But, if you do, using this while a character is grabbed will stun Ogre (providing Ogre fails a resistance check - Winter's Grasp can be resisted and Ogre seems to have a mild to moderate resistance) and cause him to drop the grabbed character. So if you have Winter's Grasp, ignore the next few paragraphs.
Since we took one melee DPS and turned him into ranged DPS, there are only two of your party members that *should* get grabbed. When Ogre grabs one, lay off your attack spells and toss the grabbed character some heals as he needs them (I hope you at least bought the first heal skill). The grab takes off health in chunks as Ogre hits the member, so don't wait to cast the heal if he's below 75%. Your heal has a cool down time you need to consider as well, so don't hold off on the heals.
The other way is to stun Ogre. To do that, you need to have a shield equipped. You can reequip a one-handed weapon + shield if you like and Pummel him, but it's really not worth it. You'll have two DPS still attacking with you (as the mage) throwing the grabbed member heals and casting Arcane Bolt while you wait on the cool down for Heal. Again, Winter's Grasp makes all the above moot.
2. Ground Pound (AoE) - This attack sometimes knocks all party members within a certain radius of Ogre down to the ground and it causes damage. It also interrupts any spells/skills that are being casted. The downed members will not get up for a couple of seconds. Ogre will still attack a downed party member. You and the Soldier (which you changed to Archer) should be well out of AoE range. Just continue to DPS.
3. Charge - This is a direct line attack. Ogre drops his shoulder and charges for a short distance damaging everyone in his path. This attack does a lot of damage. It did 45-53 damage to my party members.
4. Boulder Throw - Ogre grabs a boulder and hurls it at you. This is easily dodged and a very rare skill Ogre uses (I think I saw it three times in about 10 fights with Ogre). It's nothing to worry about, just move when you see him have a boulder in his hands.
The Grab is the one you need to be most concerned with.
And that's about it. It took me a couple of tries before I beat him on Nightmare because I didn't change my party AI. He's not hard, but party member AI makes a huge difference here. If you don't have Winter's Grasp, then I'd recommend getting it the next time you can. Nothing beats crowd control to buy yourself precious seconds.