Although Jill encounters Stage 2 Nemesis at the clocktower, it is not yet defeated and has to be re-fought in the Umbrella research facility near the end of the game.
The guide is written for the encounter on Hardcore difficulty, the highest difficulty available for the first playthrough.
Jill is recommended to have the following items with her:
a) a fully-loaded handgun; additional ammos are not required for this fight, but are highly recommended to prepare for the next boss fight following Nemesis Stage 2.
b) the shotgun + 17-20 shotgun shells
c) the grenade launcher + 18 Explosive Rounds + either 12 Flame Rounds, or 6 Flame Rounds & 6 Mine Rounds (ideally, the Explosive Rounds & Mine Rounds, due to that they can interrupt Nemesis's attacks and keep Jill from harm)
d) the magnum + 11 shells (some of the shells should be saved for the next boss fight)
At the beginning of the fight, equip the grenade launcher and keep using either Flame Rounds or Mine Rounds on Nemesis. Only reload when Nemesis is stunned; the other time Jill should keep running away from it or doding. About 6-8 shots later, Nemesis should collapse and a cutscene will play, showing Carlos arriving to help Jill.
After Nemesis gets back up and roars, it will run around the perimeter of the area, which have 8 tanks labelled from 01 to 08. Each time Nemesis does this, two to three zombies will also be disposed of into the area. Switch to the shotgun to take them down quickly. Soon, Nemesis will stop and hide behind one of the tanks, and Carlos will shout to Jill which one it is hiding behind. Quickly switch to the handgun and shoot the red square(s) on the corresponding tank, which will produce a wave of electricity to knock Nemesis onto the ground, exposing his parasite heart. Use the magnum to shoot the heart 4-5 times.
Once the parasite retracts and Nemesis gets back up, from this point on always use the Explosive or Mine Rounds to attack him so as to intercept his attacks, and the above cycle will repeat, until enough damage is dealt to Nemesis' parasite heart. It is also highly recommended to fire Explosive Rounds at it one at a time without firing too fast; the goal is to maintain its stunned animation as long as possible so that it does not utilise its long-ranged tentacle swipes.
Dealing enough damage to the heart is crucial because otherwise the cycle will repeat and the fight will not progress to Stage 3. The signal for successfully proceeding to Stage 3 will be Carlos electrocuting Nemesis directly on the tank ("I got this!") and spilling acidic liquid on it ("Not what I intended to hit, but I'll take it!")
Similar to Stage 2, but the changes will be that Nemesis will sometimes directly pounce onto Jill from the perimeter, so from this point on it is crucial to keep an eye on it while dealing with the zombies disposed into the area. Once Nemesis pounces, always dodges.
The fight will end when either enough damage is dealt on Nemesis when it is on the ground, or Carlos electrocutes Nemesis on a tank ("Surprise, asshole!") and then manages to pour acidic liquid onto it. The achievement unlocks following the cutscene afterwards.