The Colossus is a giant that specializes in support and troop destruction. He is best used as a spellcaster with lots of hit points.
Base Dmg: 53-63 which is pretty good.
Attack Speed: slow.
He has a very high STR gain, but terrible AGI and below average INT gain. Fortunately, his spells are all STR-based, so the AGI and INT are not as significant. INT is somewhat useful for a bigger mana pool and also to pump the mana regen rate. AGI pumps move speed and attack speed, BUT since Colossus has such low base values, the percentage gain is not that useful in my opinion. I would ignore AGI altogether.
I HIGHLY recommend considering Colossus as the creep version of divine wizard. Well, sort of. I was VERY frustrated with Colossus until I figured this out. Tanking was a dismal failure due to no healing or dmg reduction (besides the armor boost buff), as was trying to take advantage of his big damage...he just gets stuck and dies. And he is SO friggin slow and hard to position. Instead, just toss lots of rocks and enjoy your high HPs. There is nothing quite like eating 1k from Atk of Opp from Infiltrator and laughing about it. Note that your manapool even with an orb of the magi can only barely support Rockslide. Therefore, do not spam your secondary skill or you will be popping lots of clarity pots.
Early game, use your long range heavy damage tree-swat to last hit for cash. Toss rocks when mage and archer troops are close to death to gain money and help push. With rockslide and your great HPs you should have a decent chance of doing well with nuke-warz with Aeromancer and Divine Wizard and the like. Avoid melee wars with other melee heroes (especially AGI ones) as their rate of attack will outstrip your high dmg and HPs. Stay back from the front lines except to get a last hit and then move back out of ranged attack range.
Midgame, your rock now does significant damage against troops. Spam it as much as you can. Especially right after your mage troop zaps the enemy, so you can get the mage/archer kills for profit. You can still do well against enemy heroes in nukewarz, but they may be able to blink and avoid rocks with the wand. Still, when you wipe out their troops, they are forced to withdraw.
Late game, your rock can almost wipe out full strength standard troops in one hit! Besides getting tons of cash, this is amazing for base defense or pushing. Remember that your rock does full damage to buildings too! Try to position yourself so you can wipe out troops and hit towers as well.
 Name this build
1 - rockslide
3 - Tremor
6 - rockslide
7 - Tremor
9 - save
11 - rockslide & Tremor
13 - save
15 - Avalanche
16 - Rockslide
17 - Tremor
19 - save
21 - Rockslide & Tremor
23 - save
25 - SAVE. (dont bother with avalanche 2 yet)
26 - Rockslide & Tremor
From here, I would put any extra points from levels or crystal into Heart of the Mountain.
An alternative but more boring way to go is to take Heart instead of Tremor when leveling. This will be a bit harder on your manapool though, and you lose any chance of getting hero kills except for a lucky rock or buddy's disable.
 Items and Enhancements
Start with a few heal potions, and/or grab some heal/mana elixir from the goblin shop as you will need the mana by level 11. You will make use of heal potions throughout the game.
Your first real item should be an orb of magi. This is perfect as you dont need INT. You DO need the mana, and the regen pump. Grab this as soon as possible, as it is the key for being able to use Rockslide effectively. Your base mana regen and manapool sucks, so you really need this ASAP.
When you can, crank that Orb up first. This will significant increase your staying power. Once it is +3 or better, pick up 4 rings of Strength as these increase your Rockslide. You are aiming for about 600 damage (200 STR) so you can wipe out full strength normal troops in one rock. You can gain MAD KASH in this way and push very effectively.
This is really all you need. The Orb, 4 STR rings that you slowly pump up, and some healing potions. Optionally, you can take the Sun Talisman since you benefit from the Regen, STR, INT(for faster base mana regen and pool), and even AGI for slightly faster movespeed. I would not prioritize the Talisman though, since it will slow your STR-ramping. Unlike many heroes, the money spent in items is an investment that you WILL see returned to you as you wipe out more and more troops with rockslide and get hundreds of gold per rock tossed.
For crystal, don't bother with bonus skills as they are expensive and you dont need them. Maybe late game you can take a few and get Heart of the Mountain up some. Pump your armor first because you have no items for this. Then STR and INT 5 points each. (The INT is to pump your base mana regen rate that gets multiplied by the Orb). I would avoid melee pumps for a while since you shouldnt need it. From there, crank your STR primarily, and occasionally pump Armor.
Note: It is tempting to take orb of skill, and use the mana-regen helm instead. Dont do it! The big INT pump is pretty nice, but the AGI is mostly wasted. You dont need the movespeed boost, since your base is so slow anyways... you won't be outrunning anything. More importantly, the mana regen hat is more expensive than the Mage Orb. Getting both skill orb and mana hat is a HUGE delay to your important STR-items. The Mage Orb is much more efficient and is really all you need to maintain a good spam rate on your rockslide and decent mana pool.