Travel to the Wayshrine of Resolution in the Forgotten Vale: it's to the north of the frozen lake (with the two dragons), which itself is west of the unfrozen lake and the Wayshrine of Learning. Head directly east after teleporting - the quickest way is to gently hop down the cliff that the Wayshrine of Resolution stands on, across the river that fills the iced-over lake, and over the hill on the other side to find two waterfalls: one upper and one lower. Between the two is a flat area with a river. On the same side as the player (the western bank) is the Paragon Platform, and on the opposite side is the Frost Giant that drops the Sapphire Paragon when defeated. Incidentally, this Frost Giant is found on higher ground to the north of the one that drops the Amethyst Paragon - both are on eastern banks of two bodies of water.