Barba, the Bearded Giant, lies locked away underneath an ancient temple. When you reach the ruined floor however, his stone prison will open (via cutscene) and the malicious giant will come at the hero full force. The best strategy for this first stage of the battle is to run away. Head towards the small barriers opposite the wall Barba came out of and quickly climb over them by jumping and pulling yourself up. Barba will be close behind you, breaking each barrier when he reaches it, so be careful. Repeat this for the remaining to barriers and take shelter in the temple ruins against the wall. Barba can't get at you hear so you're pretty safe if you need to rest (in case you took some damage getting there).
If only you could stay here, protected, and defeat the colossus... Unfortunately, you must still scale the giant. Grab Barba's attention by firing an arrow at him (preferably at his chest or face) and wait inside the temple ruins. He should eventually bend down and peek inside. Seize the opportunity and run at him, jumping and latching onto his beard. Climb up to his head and the first magic weakpoint. Charge stab a few times at this position until the symbol disappears.
Weakpoint number two lies on the left side of his back. Climb down to the symbol (or try dropping down and grabbing back on; he tries hard to shake you off so climbing down can be difficult), and perform a few more charge strikes. With any luck, Barba should go down fairly easily and without knocking you off once.
Hard Mode Strategy
Well, now that you've finished off the first two weakpoints, you'll need to completely get off Barba. Drop down and hide back under the temple. If you need to grab his attention again, fire off another arrow. When he peaks inside once again, DO NOT go for the beard; instead, head for Barba's left hand (the one on YOUR right) and let him lay it flat on the ground. Climb on top and he'll get up and start trying to shake you off. He'll do this often, so fully charged strike are impossible. Just go for shart charged strikes until he finally falls.