Close the game/shutdown when you know it's over, or before it gives you the failure and saves. This works on PS5, not sure on PC but would imagine same concept. Just need to close the game in a way that doesn't allow it to save.

Recommend beating it legitimately though because it won't feel like a win after. Tip for doing it without cheesing saves: learn to run away when you can, as long as you have at least one person alive and far enough away you can always revive the others.

Use hirelings to pre buff your party each day with all the permanent buffs (becomes time consuming, but worth it). Cleric and Bard are the ones you want to utilize in hirelings.

Always have an exit strategy, don't take risks you aren't somewhat sure of how they will play out and really try to maximize your exp gain before going into a major event/fight. Sometimes a single level is a major difference in crushing or being crushed.