Uroboros Wesker

Chapter: 6-3 · Hangar and Volcano
Where: The volcano, by the bomber wreck (items lie near the wreck before you walk on). Phase 1 separated: Chris below, Sheva across the gap. Phase 2 together, on the circular high ground.

Weak point

The orange spot on his back (visible when he raises his arm) and, in phase 2, the chest core, which opens after the whirlwind. A Rocket Launcher round on an exposed weak spot kills him.

Weakened by the PG67A/W overdose, Wesker takes the Uroboros himself: tentacles on both arms and metal debris fused to the right one. His arm swing kills on contact or knocks you into the lava.

Phase 1, separated: Chris fights below and pushes a boulder into the lava so Sheva can cross. Phase 2, together: the back → whirlwind → chest cycle. On weapons the sources clash: IGN wants a maxed magnum (about six well-placed shots) and Fandom says magnums do 60% less to him and recommends a rifle or Grenade Launcher. The rifle is the safe middle ground.

Reward

The end of the game. The Diamond (Marquise) (₦5,000) is picked up at the start of the fight, as Chris only.

Difficulty: on the harder settings you need full health to restrain him (the hold drains your health and can still fail). If it fails, Chris falls and Wesker impales Sheva. Fandom says that on higher difficulties he takes only 10% damage outside his weak spots (single source).
Attacks: arm swing (death or into the lava), ooze projectiles (run or follow the prompt), raised-arm slam (dodge prompt) and the phase 2 whirlwind.
Without the Infinite Rocket Launcher, replay 3-1 to stock free rockets.
With the Diamond (Marquise), Wesker follows you there and can kill you in one hit: have your partner cover you.

Step by step

First (Chris only): turn around and run up the slope without crossing the rock bridge; shoot the bottom edge of the off-colour outcrop on the right so it falls into the lava. After your fall, turn left: Diamond (Marquise).
Phase 1, Chris: go up to the circular high ground. When he raises his arm to slam, dodge with the prompt, flank him and shoot the orange spot on his back.
Phase 1, Sheva: shoot his back from range to bait him to her side.
After enough damage he mutates his other arm and jumps to Sheva's side; her bridge collapses and she hangs on (mash). Chris shoots Wesker to keep him off her (about 30 s). If Chris already hurt him before, she climbs up with no mashing.
Chris runs down around the island to the boulder in the south-east and follows the prompts to push it into the lava. Start only once Sheva is safe, but don't dawdle: if Wesker kills her, you lose. Checkpoint as you head back up to the high ground.
Phase 2: shoot the back core when he raises his right arm → he flinches into the tentacle whirlwind (instant death up close): get away → the chest core opens: shoot it until it closes. Repeat.
To keep your distance: 2 handgun shots to the head stagger him back a step or two.
Finish: when he kneels after a chest hit, Chris restrains him from behind (mash), Sheva stabs the chest and Chris impales his back. Or keep shooting the chest until he dies.
One-rocket shortcut: right after Chris's fall, when Sheva shouts for you to get to higher ground, run straight up the hill, go left to the farthest point and aim down the slope: he walks up with the orange spot on the back of his left shoulder showing. One rocket ends the fight (only the final QTE is left).
Final QTE: he grabs the helicopter. When Chris says "Take this, Wesker!" and looks through the scope, press the prompt to fire the two RPGs. Missing it means death.