Again, no nukes. As Lennon said, just give non-nuclear war a chance.
Then I suppose war gasses are out of the question? 
We would not do it at all.
The only reason Iwo Jima was needed was as an airfield for the short-ranged planes of the WWII-era. With todays “military technology”, we have much longer planes, so we don’t need it at all. We could have done the bombing of Japan from Pearl Harbor, or even San Diego.
So Iwo Jima would have just been bypassed, and left to ‘wither on the vine’, while we prevented Japanese ships from supplying the provisions & ammunition their troops needed. We could proceed to invade the Japanese homeland and force them to surrender, without worrying about isolated islands they still held out in the ocean.
This was the basis of the “island-hopping” strategy in the Pacific. We got to pick-and-choose where we would fight: just bypass the heavily defended islands and attack at other, unexpected places. But in some cases, this wasn’t possible – logistics & travel ranges made it obvious (to both us and the Japanese) that we would have to take certain islands, so they were heavily defended, and lost a lot of troops taking them. Iwo Jima was one of these. Though there were some American officers who didn’t see it as so critical; they advocated skipping it and attacking elsewhere.
Two thing come to mind that we do have that weren’t used in WWII.
Napalm, and Fuel/Air Explosive [check out the pics at the bottom of the page]
Hereis a 15,000 lb BL-82 Daisy cutter.
Drop one of these near the bunker, and the overpressure will kill the the people in the bunkers.
They’d use a lot of thermobaric bombs. Thermobaric bombs suck the oxygen out of the victim’s lungs and kill people way down inside.
However, if Iwo Jima were invaded today, the defenders would ship images and videos of masses of bodies and body parts around the world, claim atrocities were taking place, show the bodies of children and women, and the world including the U.S. citizenry would turn against the invasion and it would have to withdraw.
Took the words right out of my mouth ,Fuel Air Explosive would be ideal,but I think its been made illegal under one of the conventions ,not because of its explosive effect but in case the targets inhaled it before it was ignited.
I might be wrong though.
What about the Gand Slam?
Grand Slam, even.
Assuming we attacked the island at all, we’d strike at night, and clai large swaths of it before the defenders could respond effectively. It’s much harder to call in artillery strikes at night against a moving enemy.