Severe hypercalcemia of malignancy is a fluid problem before it is a bone problem. High calcium blunts the kidney's response to ADH, producing polyuria, and patients often vomit, so they arrive markedly dehydrated.
The first move is therefore vigorous isotonic saline resuscitation. Re-expanding the intravascular space restores glomerular filtration and enhances urinary calcium excretion, dropping the serum calcium within hours.
Only afterwards do the antiresorptives matter: a bisphosphonate such as zoledronic acid shuts down osteoclasts but its effect is delayed by $2$-$4$ days, and calcitonin can bridge that gap. Loop diuretics are reserved for the volume-replete patient who develops fluid overload - giving them to a dehydrated patient worsens things. Searching for an exogenous calcium source is pointless when the driver is tumour PTHrP/osteolysis.
\[\boxed{\text{First step} = \text{IV isotonic saline (volume expansion)}}\]