I tried an experiment on Fairy Heart v2.4: on a fresh copy of the game (no save data), I tried to see how far I can progress by upgrading only the Strength stat. By stage 6, the difficulty climbs steeply: besides the increased attack damage, the lumbering monster outranges her melee attack, while the running one tends to chase her down and prevent any form of "breathing space". Finally, both their attacks do damage even while she's down, and reset the knockdown timer every time, making recovery difficult.