Start the game on Easy and play until you have gotten past the first Djin. Upgrade at least 2 things to the next level, then start a new game on Normal. When the game begins, switch the difficulty to Easy and then follow the story till the first Djin again. Just after that area you will see that your upgrades from the previous game will appear and all the XP in the levels have been reset (Monsters and Sarcophagi). Repeat as many times as desired and you can have all the available upgrades; and you can save up upgrade points for the later unlocked ones.
When you reach Rekem, there is an area where you fight three Glowing Sand Creatures (the ones that spawn other Sand Creatures). Use your power slash to kill the Sand Creatures that they spawn. After you have a couple of hits, run around the arena and repeat the process until you unlock the achievement or trophy.
Search the following locations to find all ten Sarcophagi. When you come near a Sarcophagus, you will hear a sound and see their blue orb glow:
Close to the exit of "The Stables", climb to the top of the octagonalshaped room.
In "The Fortress Courtyard", jump to the platform under the circular switch you have to activate to progress through the level.
After the spiked floors in "The Fortress Courtyard", drop off the main path and slide along the series of ledges to reach the Sarcophagus.
Near the exit of "The Prison", activate the switch to open the gate. As you run past the traps to roll under the closing door, but before you exit the prison, go to the right to climb the wall. Then, go right again.
At the start of "The Sewer", when climbing the frozen water spout, jump towards the beam. Use the frozen water spout again to reach a second beam above the first one. Then, jump over a gap to reach the Sarcophagus.
Near the exit of "The Sewer", walk along the side path on the left. Then, freeze the water to swing across the gap and reach the Sarcophagus on the other end.
Enter a room in "The Royal Chambers" with a water spout on the left with switches to its sides. Then, freeze the water and use it to jump towards each switch. The Sarcophagus is behind a door that opens after activating both switches.
In the middle of "The Royal Chambers" there is a room with a waterfall on both sides and two accompanying water spouts. Freeze the water, jump up to the second water spout, and use it to jump towards the waterfall. While in midair, immediately unfreeze your ability so you will go straight through the waterfall and land on a ledge. Then, climb up to find the Sarcophagus.
Near the exit of "The Royal Chambers", jump on a ledge to the right of the water wheel room and proceed on the ledge without freezing the water. Then, perform a wall run up from the ledge to reach the wall. Jump off the wall to reach a pole. Jump off the pole to another ledge, and move left across it to find the Sarcophagus.
At the start of "The Throne Room", climb the wall on the left until you reach the balcony to find the last Sarcophagus.