Dungeons 3
60 Achievements
1000 XP
Band of Brothers
Don’t lose a single creature to Tanos in battle in the mission “Everything Has An End…”.
50 XP
4.0%
How to unlock the Band of Brothers achievement in Dungeons 3 - Definitive Guide
Now, a safe way to defeat Tanos is to just use Thalya and the 3 Titan creatures (Ogre, Pit Fiend, Grave Golem). So take the level slow, build up your dungeon and research each Titan, research their Lecture Hall upgrades and train them in the Arena as much as you want. Also fully upgrade Thalya.
Save your game before the fight!
I defeated Tanos with the following army (the two tomes are found above ground):
Thalya - Level 11 - 10/10 upgrades (with health regen tome)
Ogre - Level 8 - 2/2 upgrades
Grave Golem - Level 7 - 2/2 upgrades
Pit Fiend - Level 3 - 2/2 upgrades (with life leech tome)
Make sure you use Thalya's Frost Nova ability when Tanos puts up a fire shield as that makes him Immune. It's a fairly easy fight but try to micro Thalya out of Tanos' flame strike circles as she's still the weakest.
Update: Thanks InkyPaws for finding out that Thalya getting killed ny Tanos also locks you out of the achievement.
This is done in the "Everything Has An End…" level. The description is slightly misleading as it does not mean you cannot lose a single creature throughout the session. It only means that you do not lose any creature when fighting Tanos directly. Tanos is waiting in the centre of the level and won't do anything until you attack him. Once attacked, you cannot lose a single creature. This can be tricky if you're using normal creatures so bring Titans. Scattered around the map are 2 heal-over-time spell books, which will come in handy and should be given to the Pit Fiend and Ogre as they are not as tough as the Grave Golem.
During the fight Tanos will lob firebombs occasionally and it is this attack that will likely cause deaths but will hardly damage Titans, though the Pit Fiend has the highest chance of dying if he does not have the heal-over-time spellbook. Also, since there's no time pressure, make sure to research all the Titans' upgrades.
For the actual fight, you want to bring as small an army as possible for easier control, especially when dodging Tanos's firebombs. All you really need are the 3 Titans which will be on automatic just attacking Tanos. For support, bring Thalya and 2-3 maxed Naga to heal the Titans. During the fight, control only Thalya and the Naga (press + ), just pay attention to where the firebombs will land (denoted by a small red circle) and quickly move out of the way. When ready, use the Frost Nova attack which somehow lowers his defences enough so that your Titans do more damage to Tanos. If you see one of your Titans get low on health move him away from Tanos, let Tanos auto-target someone else then resume fighting. Just make sure to keep your Nagas alive as they will be the weakest.
If you have kept moving and kept everyone alive, once you kill Tanos the trophy is yours and likely the platinum as well.