I mean round up the damage that gets transferred, but not the portion of hp lost.
I.e. the card would be capable of dealing .5 HP worth of extra damage when used on a creature with odd HPs
I.e. if you use it on a creature with 3 HP, the creature is left with 2 HP and this card deals 2 damage.
If used on a creature with even hp everything is normal.
... I admit its a bit weird, but welcome to discrete arithmetic.