You apply power/toughness changing effects in a series of sublayers in the following order: (a) effects from characteristic-defining abilities; (b) effects that set power and/or toughness to a specific number or value; (c) effects that modify power and/or toughness but don’t set power and/or toughness to a specific number or value; (d) changes from counters; (e) effects that switch a creature’s power and toughness. This card’s effect is always applied in (b), which means that effects applied in sublayer (c), (d), or (e) will not be overwritten; they will be applied to the new value.

Note that the duration of Halfdane’s ability is until the end of your next upkeep. If Halfdane’s ability doesn’t get a chance to resolve (because there are no legal targets when it triggers, or because it’s countered), then the effect from the previous turn’s ability will continue to work through the rest of that upkeep step. Then that effect will wear off, and Halfdane will revert to being 3/3, its printed power and toughness (unless some other effect is modifying what its power and toughness is).