Modern Emerald Hard Mode: Every Change, Ace by Ace

Hard mode is not just higher levels. It changes the maths of every battle, and it rewrites specific boss Pokémon so that the vanilla answers stop working. If you are going to play it, know what is actually happening.

The five global changes

Your Pokémon take more damage. Attacks against you get a 1.25× multiplier that scales with your badge count: 10% with one badge, then 15% with three, 20% with four and 25% with five. This is why early Hard mode feels brutal and why a five-badge team can still lose to a route trainer.

EXP gain is cut by 40%. There is a "Hard mode Experience" option that removes the cut, but the hack's own documentation says not to use it — the 40% was hand-tuned.

The Gen VI EXP. Share gives only a quarter of the experience to the rest of the team, so you cannot train a whole party by accident.

Badge boosts are disabled, which removes the free stat advantage you normally accumulate.

You are limited to four items in trainer battles, not wild battles — and held berries count towards the limit. The hack's notes describe this as the change that catches people out most.

Wild Pokémon and trainers both scale levels according to your badges. That is the main source of difficulty, and it is why the level cap tables shift upward in Hard mode.

Every ace that changes

Norman's Slaking without Truant is the single biggest change in the game — it attacks every turn instead of every other turn, and at that point in the story you have no reliable Fighting move unless you have been planning for it.

Juan and Wallace both set rain, which turns Water moves into a real problem and halves your Fire damage for the whole battle.

Also worth knowing

• Rayquaza is level 50 in Hard mode instead of 70, and legendary stats are boosted while you fight them. The catch rate is also lower for some of them, with Mewtwo, Mew, Ho-Oh, Lugia, Celebi, Kyogre, Groudon, Rayquaza, Deoxys and Regigigas dropping from 3/255 to 2/255.
• Frontier bans — if enabled, legendaries are banned from the Battle Frontier, and the ban depends on the difficulty you picked.
• Hard mode locks Battle Style to "Set" while "Lock difficulty" is on, and it cannot be changed until you beat the game.

Change list from the official Modern Emerald documentation (Hard mode tab); damage scaling, EXP cut and item limit values from the same source. Level caps read from the game's src/caps.c tables.