NOTE: Credit to this guide goes to user "Bowzertv" from the forums
The reasoning behind this build is to maximize survivablity while still having strong AoE damage for farming. Bulwark of Steel and Osmium Skin are the two skills which allows Colossus to survive within fights. It is very important to max these two skills as soon as possible. The next important ability for this spec is Spinning Fists. Spinning Fists has the maximum potential for DPS. This is where all your DPS will come from as Colossus. So get ready to play a WW barb all over again. This brings us to our next ability which is Sustaining Aura. This aura allows Colossus to have "decent" spirit regeneration which fuels our Spinning Fist addiction. Without this aura playing this build would be very painful, I do not recommend it. We can pick up defense, tenacity, and damage within our gear. Now to talk about our armor regeneration. Osmium Punch is an amazing ability and is a must have as Colossus for any spec. This ability allows us to regenerate our armor which will soak A LOT of damage in combat. It also does a decent amount of damage when our spirit is too low to spam Spinning Fists. Now to wrap up my logic with this build. I put 1 point into Osmium Charge to move quickly around the map and to get out of dangerous situations (extra damage mobs). I also put only 1 point into Break The Chains and Heroic Challenge. These skills are decent for certain situations so it is nice to have them if we need to get mobs off our teammates or to get out of snares and slows. The remaining points I spend in Metallic Warrior because who doesn't like to see mobs die quickly.
The next thing I would like to cover is Colossus' DPS "rotation" for this build. It is very simple but the damage output is very nice. Use Spinning Fists until you are out of spirit. (Circle around the mobs while spinning for maximum damage to every mob around you) Once you are out of spirit, you will auto attack with Osmium Punch until our spirit returns to a good amount. I usually just wait until it is full again but it changes in certain situations. I will recommend to acquire a spirit on hit weapon because any spirit regen is nice to have as Colossus. Plain and Simple.
The last thing I want to discuss is how I am currently gearing my Colossus. If I had to use one word it would be DEFENSE. I stack HP, dodge, and of course defense. Dodge is the most important stat to have in this crazy game because of how hard bosses and mobs hit. Dodging attacks will allow more time for our armor to regenerate to full before that next big attack hits us. Attack speed is also nice to have for our armor regeneration but it is NOT a must have. I strongly recommend to acquire 2 Starstone Tooths. The stats are perfect for most tanks but for Colossus especially because the heal on hit works with Spinning Fists which is our fastest attack by far. The medal I like to use is the Juggernauts because it has a good amount of HP, defense, and being immune to damage for 5 seconds or so has saved my life a few times.