Heart of Africa is the most valuable treasure in Resident Evil 5, worth ₦10,000 each. If you're looking to upgrade your weapons, this is the perfect item to farm. Here, I have already farmed quite a few of these gems.

Note that this treasure won't drop unless you defeat Wesker under 7 minutes, so you must defeat him before he decides to run away. He will run away earlier once you hurt him enough. Doing this will also net you the Bad Blood achievement.

I apologize for bad continuity between these pictures since it's really hard to take a screenshot during these very time-sensitive events. But I hope you can get the general idea of what to do.

Here are a list of items you will need:

• A fully-upgraded Magnum. Any Magnum will do, but I prefer upgrading the S&W M29 since fully upgrading it unlocks a new Magnum.
  
• Enough Exchange Points to unlock the infinite ammo for the Magnum (15,000 pts). You can farm points by playing Mercenaries mode or the Mercenaries Reunion. Recommend using Wesker, Excella, or Rebecca Chambers.
  
• Not really needed, but Longbow for Sheva since it's really strong. Unlocked by fully upgrading the S75 rifle, and buying it after finishing the game for ₦50,000.
  
• Some healing items just in case.

Load up Chapter 5-3 and play normally. Use your infinite Magnum to make things easier. After the section where you lower a drawbridge to enter an old ruin, you will be at the Monarch Room. Here, you will face Albert Wesker and Jill Valentine. Have your Magnum equipped before entering the room.

After a cutscene Wesker will announce that he only has 7 minutes to play with you. Run past him, and after you reached quite some distance, turn around and shoot him in the back. I usually turn around while I'm standing on the colored tiles like the one he's standing on in this picture, there's another behind him. Be careful about accidentally hitting and killing Jill.
Note that this method may sometimes not work, either you're not in the right position or maybe just chance. In that case, Wesker usually walks toward you, but Sheva may shoot her arrows into his back, staggering him and creating a similar "window" for you to attack.

Wesker will turn around and rush towards you. Be prepared to press the button prompt when it appears. It is always A + D. Chris will counter Wesker's attack.

Wesker will fall down, but get up very quickly. After he gets up, shoot at his lower torso, or if you're feeling naughty, his crotch. Even though you're in front of him, usually he cannot dodge. These shots will stagger him. After he is staggered, he can take one more shot, either by you or Sheva, before he returns to charge at you, repeating step 2.

While he's staggered, you'll get a prompt (F) to start a beatdown on Wesker. The combo of this beatdown is always F - F - V - F - F + V.

After the beatdown, Wesker will fall to the ground. He doesn't get up as quick as after he's countered, but if you shoot him, he will instantly get up, and he will rush to you again, and you must counter him again, repeat step 2.

Keep on repeating steps 2-5 to "stunlock" Wesker, and keep beating him down to deal damage. It seems that continuous stunlock can actually prevent him from escaping early, and can take somewhere between 5-10 times. I recommend shooting your Magnum at him and use the combo when you need to get some momentum or to reposition him, preferably with his back facing Sheva, since Wesker will dash to you if you shoot him. Eventually, he will run away and the next phase begins.

You will start on top of the stairs with Sheva and Jill. Be careful, her gunfire is quite deadly. You can try calling out to her which may confuse her and make Sheva grab her, immobilizing her for a while. Run to the left, and shoot the left pot on the end of the hallway since it's another treasure, the Emerald (Brilliant). It's worth ₦2,500.
Then jump down, and head to the space between the stairs. There's the Heart of Africa. Take the 2 Green Herbs to your left and right and combine them to provide an extra, decent healing item.
Finally, quit and save your inventory. Both treasures will be in your inventory after you quit. When you click Continue in the menu, you will begin at the start of this area, and you will have to fight Wesker again.

I should note that in some cases, Sheva will initiate a beatdown on Wesker if she's closer to him. Just keep firing at him just in case. I don't know if the damage counts or not, but it sure is fun and satisfying. Besides, you have infinite ammo anyway.
There's also some more treasures in this room and a Magnum gun. They are located in the second floor, in a room with coffins. The Magnum and 2 gems are inside the coffin which requires 2 people to push, and another gem in a small vase in the room before. If you're unlucky enough to have Wesker break away from your stunlock and kick you through the blue door, you may move here and collect the treasures before fighting him again.

There is another simple way to defeat Wesker quickly, and that is by stockpiling rocket launchers from Chapter 3-1. When you reach Wesker, shoot the rocket at his back, and while he tries to hold it, use another gun to shoot at the rocket. You can see the location of the rocket launcher here. Or alternatively, beat the game under 5 hours on any difficulty (preferably easiest) to unlock the infinite rocket launcher for the fastest method.

Thanks for reading this guide, and I hope it can help you farm money and defeat Wesker. If you know a quicker way to defeat Wesker, maybe by using a different Magnum, feel free to let me know!