Paying cumulative upkeep is always optional. If it’s not paid, the permanent with cumulative upkeep is sacrificed. Partial payments of the total cumulative upkeep cost can’t be made. For example, if a permanent with “cumulative upkeep {1}” has three age counters on it when its cumulative upkeep ability triggers, it gets another age counter and then its controller chooses to either pay {4} or sacrifice the permanent.

Ronom Hulk can’t be targeted by snow spells (though there are no snow instants or sorceries) or abilities of snow sources, it can’t be enchanted by snow Auras, it can’t be equipped by snow Equipment (though there are none), it can’t be blocked by snow creatures, and all damage that would be dealt to it by snow sources is prevented.