To get the Heart Of Africa treasure you must do a set number of damage to Wesker during the first fight with him and Jill. The best way is to have the Infinite Magnum. You will also get the achievement if it is your first time. After you defeat him and start of the Jill fight, the treasure will be between the two curved staircases. Also, do not forget to get the gem in the southeast corner of the room you enter in the crate, and the two in the tombstone upstairs, and the one nearby in the little pot.