This guide will help you to beat Final Boss on Hard Mode using only ammo that is found in last storage (B9) before entering Boss Arena.
It will save you from replaying the entire game and saving heavily on ammo, like most strategies for Hard Mode suggest.
It includes a video in last section.
It is strongly recommended that you read through the entire guide before watching the video, as it explains everything in detail.

If you like the challenge and started with HARD mode on your 1st playthrough, without any assistance or guides (like me) - then it is very likely you will run out of ammo after repeated showdowns with Prototyrant and Queen Leech in the last part of the game.

Officer Chambers does not give up, so neither are we.

General strategy can be divided into three main sections :

INVENTORY

Storage (B9) on Hard Mode contains next items :

- 30 Handgun ammo
- 6 GL rounds (regular)
- 7 Shotgun shells
- 2 First Aid Spray
- Green Herb
- Red Herb
- Ink Ribbon

Assuming we have 0 ammo after difficult Queen Leech fight in Incinerator we should divide the items between Billy and Rebecca like this :
Only one consumable (1 healing mix) present in inventory is a leftover from previous fight. All other ammo and healing is from Storage B9.

Bear in mind that items can be hard to exchange between characters during the fight, so we have to plan accordingly who will carry what. There is always an option to DROP&PICK UP, but it's slow and dangerous.

Notice that I don't even carry 30 Handgun ammo and 7 Shotgun shells, but 28 and 4 (screenshot from video below). And it is still doable.

FIGHTING STRATEGY-OPENING PHASE

We will control Billy directly and set Rebecca to Team and Attack modes. Rebecca will only use upgraded Handgun and Knife.
Very important is to use Billy's ammo and switch to knife as soon as possible - that being said we must preserve 2 GL rounds for fight's most critical stage.
Always shoot the Queen at point blank range, because that's how weapons do more damage (even the Handgun).

After switching to knife, keep stabbing the Queen fast, most preferable section is the Tail, because she will often waste time to turn around and face you. If you start running in circles while she turns, she could easily be confused and start following you endlessly while Rebecca is unloading ammo on her.

AI controlled Rebecca sometimes shoot slowly or do not shoot at all. If this happens, change control to Rebecca and quickly shoot 7-8 times at Queen, then switch back to control Billy. Pay attention to Rebecca's clip and add ammo manually in inventory, or she will lose valuable time during the fight to reload.

Control your health levels!
Do not use health if your status has not reached Danger, because you probably won't have enough to finish the fight. That means you must check your health levels almost every time after being hit. Needles to say, after reaching dangerously low health levels, heal immediately.

Queen Leech has 500HP.
Depending on range, Handgun deals 2-8 DMG, Shotgun 4-31 DMG, Grenade Launcher 29 DMG and finally our best friend The Knife whooping 8 damage. That's right - Knife deals as much damage as Handguns maximum damage Point Blank range. That's why we don't need infinite Handgun ammo drops in last Boss arena like in other RE games. We're gonna Knife the bastard!

If we got all (or most) of our shots in, estimate is that Queen will still have around 100-120 HP depending on range of our shots that hit her. That means that we only need around 12-15 Knife stabs to bring her to Final Phase. If you started stabbing her on time while Rebecca shoots, and followed a specific fight dynamics provided in video - this shouldn't be a problem.

FIGHTING STRATEGY-FINAL PHASE

Final Phase begins when Rebecca start opening valves and her character is no longer under our direct control. However, we still control her inventory and need to maintain her health levels! As soon as the Phase starts, we can even exchange items if needed for one brief moment.

We have to understand the Queen's strategy to be able to effectively counter it without ammo.
Queen will prioritize Rebecca, so our job is not to kill the Queen- but to aggressively harass her and make her start turning towards us (which will cause her to lose valuable time).
That is entirely possible using only a Knife.
Only exception is the beggining, because the Queen is very close to Rebecca and we need to shoot those 2 GL rounds that we saved from Phase 1 as soon as the fight starts.

After that, tactic is straightforward - stab repeatedly until Queen turns, then retreat to save Billy's health. Repeat.

Everything that has been explained previously is shown in video.

In video i used even less bullets (28 Handgun and 4 Shotgun - instead 30 and 7) because i wasted some of Storage B9 ammo by running back and forth through the base before going to Boss fight.

I hope you managed to finish the game with help from this guide.

Remember that your performance will improve with repeated attempts. You will slowly adapt to Boss movement and attack patterns. In time you will notice how easier it becomes.
Keep trying and don't give up!