You need to gear the little ball of death like you would gear yourself as a TANK. You need Aim, Endurance (those 2 are a given) and then, now get this, you need DEFENSE stats (defense, shield / absorb). As one poster above stated, ya'll think theres an issue with the little fella because hes tanking, your dps'ing, and the little fella aint gettin healed then he dies... well um, duh?
For healing, you need to have the healy gas cylinder on, then if you spec'd into healing, you use kolto missle on em, and your scans, and bust some dps on the critters. You can also use quick shots on him to do some fast healing and get more ticks for the healy cylinder proc.
If your NOT healy spec'd, then make sure you do toss some heals on him, use them scans! also remember, you have concussion missile, USE IT! it will knock one mob out of the fight for 1 minute just make sure you dont use any AE or have blizz with his AE's turned on.
For those who dont know how to turn on/off your companions abilities: first, make sure you have all your action bars turned on (go into preferences / user interface / then turn on all the action bars) then in the same are (preferences) you can select WHERE your pets action bar will go (Left, Bottom Middle, or Right) make sure that you dont have any abilities in the area your gonna slap his bar into, the companions bar will overwrite them when its active.
Next you hit the tilda key (~) top key right above the tab key on the left side of your keyboard. this brings up your companions ability bar. If your gonna CC, make sure the AE abilities are turned off (they will have a white button on them in the middle of the ability to tell you the ability is turned on). And there you have it! you can manually tell your companion to use a ability that is turned off you just Left click the ability and off it goes! this can be usefull if say you send blizz into a blob of critters, have him light of an AE, then toss a concussion missile on the elite in said pack, and he will keep agro. \
Hope this helps yall out, have fun and let the little ball of doom loose and just laugh at the carnage! Nothing says we just wooped your *** like a 3 foot tall walking robe with a rocket launcher standing on your smoking corpse!