Here is my final result on Hard; played NTSC-US PS2 version on a Slim PS2:
The major difference is the Leonard battle where Press Continue took 13m 38s (around 18m real-time) compared to my 5m 52s. I used a different strategy using the three-slash combo rather than doing a forward-thrust.Action Level: Hard (5/5 stars)
Riddle Level: Hard (5/5 stars)
Clear time: 3h 09m 39s (5/10 stars)
Split worm kill time: 16m 05s (0/2 stars) EDIT: Got it down to 9m 01s on Extreme 1 using alternate strategy.
Missionary kill time: 8m 34s (0/2 stars) EDIT: Down to 6m 13s using slash combo.
Leonard kill time: 5m 52s (1/2 stars) EDIT: Down to 3m 48s
Memory of Alessa kill time: 12m 59s (0/2 stars) EDIT: Down to 9m 27s using alternative slash-and-thrust combo. Second try: 8m 26s
The god kill time: 45m 23s (0/2 stars)
Clear number: 1 (1/5 stars)
Save & continue number: 10 (3/5 stars)
Item number: 63 (12/20 stars)
Defeated enemy by shooting: 0 (0/15 stars)
Defeated enemy by fighting: 5 (1/15 stars)
Total damage: 939 (4/5 stars)
RANK: 37/100 stars (3 big stars and 7 little stars)
When hit from the back, Leonard may try to move forward, or turn toward Heather. If Leonard is moving forward, he will steadily move away from Heather so you won't get as many as hits in. While it's possible to move forward with him, holding an extra button could be cumbersome. Optimally, if Leonard turned toward Heather, he is positioned in such a way that he is not moving away as much. Heather can get many more hits in that way. In either case, Leonard cannot seem to attack as Heather is slightly quicker with the knife and cancels out his motions. In the interim, Leonard will perform the swimmer's diving motion.
If Heather is able to connect while Leonard is doing the diving motion, Leonard will fail to dive and be in an upright stance once again. Heather can stun-lock him this way and connect a few more combos. Eventually, Leonard falls onto his knees, and he swims off afterwards.
Leonard is able to break away if the combo slows down due to Heather's stamina being depleted. He can also break away if your attack started a few frames too early or too late; This can happen when you're constantly mashing on the X button and your finger glitches. Leonard will have that split second to dive and Heather will take damage. Heather should have full stamina or close to it when she engages Leonard. If you don't have an ampoule equipped, one strategy is not to run as much and let Leonard come to you. This is the strategy I used. You can also equip the health drink and use it (R3) when her stamina runs out.
As this is in Hard mode, Heather should stay away from walls as her knife attacks can deflect off of them and cancel out the three-slash combo. This is one of the worst things that can happen as Heather becomes vulnerable in this state. As her deflect animation is playing out, Leonard will start attacking. Heather will take a hit before she is allowed to move.
If your first instinct is to run away, the water will slow Heather down just enough for Leonard to connect another hit. If you're really unlucky, Heather will crash head-first into the water and give Leonard time to storm towards her. A better strategy would be to block and let Leonard go back into the water.
Anyway, give it a try if you want a new challenge (or have nothing better to do). The god battle at 40+ minutes is a bit excessive, in my opinion. I'm not even sure the knife can connect when she is in her upright position..., so a lot of time was wasted just waiting for her to come down.
EDIT:
On Extreme modes, Leonard is trickier. When he turns toward Heather, he does an arm swing which can connect in Extreme. Also when Heather engages him from the front, his arm swing isn't cancelled as much as it was on Hard mode. You can try timing this and block the arm swing. Once you have blocked the arm swing, you can continue with your slashes.
On Extreme X:
Clear time: 2h 16m 03s
Split worm kill time: 16m 03s
Missionary kill time: 6m 55s
Leonard kill time: 4m 41s
Memory of Alessa kill time: 11m 53s
The god kill time: 42m 08s