The Istari were "clad in the bodies of Men, real and not feigned", unlike most Ainur who were inherently spirits and used bodies "only as we use raiment".
However, Morgoth became permanently embodied in his attempt to dominate Arda's physical matter, and the same happened to the Balrogs.
Something of the sort probably happened to Melian, given that she was able to have a child (and it's supported in late writings in History of Middle-Earth), but her return to Aman is somewhat problematic given that assumption. Huan was likely also a Maia, and was physical enough to be killed.
But -- was the embodiment of the Istari the same as these other embodied Ainur, or was it more limiting?
Saruman gets killed with a knife - it seems that that probably wasn't an option in the case of Morgoth or Sauron*, as nobody suggests trying to poison them or sneak an assassin into Angband or Barad-dur. (Beren and Luthien had a sleeping and unguarded Morgoth right in front of them and didn't try to cut his throat or anything.) On the other hand, while Gandalf dies in the battle with the Balrog, he seems to survive much longer than a normal human would - more like the Balrogs themselves, who are killed in physical combat, but only with great difficulty.
*OK, Sauron may not have been permanently embodied, but it took him at least a thousand years to recover last time he was physically killed.
TL;DR: Are the Istari more vulnerable to physical damage than other permanently-embodied Ainur? If not, why isn't this exploited more?