Stage 3 Nemesis is the final boss of the game and follows almost immediately after Stage 2 Nemesis in the Umbrella research facility. Unlike his Stage 1 & 2 versions, the fight is designed to be more like a puzzle than a conventional head-on engagement.
The following guide is written for Hardcore difficulty, the highest difficulty available for the first playthrough.
At the start of the battle, turn around and interact with the anti-B.O.W. railgun behind Jill. Press RT and Jill will fire a beam at Nemesis. However, the weapon now needs cooling, and 3 of the power sources fail to put themselves back into positions. The objective now is to target Nemesis's weak spots: sacks which are pink in colour. Shooting them all would knock Nemesis to the ground, giving Jill time to push 1 power source back before Nemesis gets back up.
Nemesis has 3 sacks at the beginning, each of which takes 3-4 handgun bullets to destroy; the magnum can always destroy a sack in one shot. After 1 power source is restored, Nemesis will spawn 4 sacks, and after 2 power sources are restored, he will spawn 5 sacks. Therefore, to finish the fight successfully, Jill should have with her at least 36 handgun bullets or 12 magnum rounds if no shots are missed. There are, however, some handgun ammos in the room in case you need them.
The 3 power sources' positions are (when facing Nemesis): 1 to the left, and 2 to the right. They are highlighted in red and thus should not be too difficult to notice. Pushing the right power source nearest to Nemesis first is recommended, as it takes the longest amount of time for Jill to run to it and back away to prepare for the next round of fight. The power source nearest to the anti-B.O.W. railgun, on the contrary, is recommended to be restored last.
Nemesis's main attacks include swiping his two long arms twice, slamming them twice, and hammering them on the ground towards Jill. These attacks on Hardcore difficulty are fairly slow and can be perfect dodged.
When all 3 power sources are pushed back into positions, pick up the anti-B.O.W. railgun again and press RT again to fire a second time at Nemesis, which then will be completely vulnerable. Finally, walk towards it and press RT a final time to kill Nemesis once and for all, unlocking the achievement.