I'm not sure myself but I'd say on hard, you'd probably want to make sure you grind a lot so you have Ultimates/Megas early and rotate them as needed if it's a new game (aka your party memory is still low since NG+ keeps your party memory). Also Make sure you have an Ultimate/Mega that has a DEF piercing move (is Okuwamon/GranKuwagamon) for each eater fight.
Character Reversal would obviously be very useful if you need to switch the strengths and weaknesses of a Digimon (I don't know who else has it but PlatinumSukamon has it at around level 25)
Also making sure you have a wide variety of Data, Vaccines and Virus types would help for all non-eater battles. Take full advantage of weaknesses and if you know they are weak to both a type and his special attack element (such as Fire Mega Burst on the GeoGreymon line against a plant type like Palmon), use it so you can get a sweet 3.0 dmg instead of the usual 2.0 dmg when your enemy is weak to your type.
Most of these are common sense tips (imo) that you can ignore somewhat in normal mode but based on previous posts about hard mode (since I've not done it myself yet), I'd say many of these would be mandatory for hard mode. Take full advantage and exploit as much as you can during boss battles and the like.
Also keep a supply of items on you since you may need them a lot more in hard mode.
And remember, you can always turn the difficulty down if you think it's a bit too much

.
That's all the tips I can really offer you. Good Luck!