Regarding your point, heavy weapons don't only include armored vehicles. Even with air cover to prevent 90% of their armored vehicles and artillery from being used, heavy weapons like recoilless rifles, heavy mortars, doushkas (Soviet 12.7 mm equivalent to the .50 cal M2), 25mm SPAAs mounted on trucks, old anti-tank guns and a shit-ton of RPGs are things that can slip through due to smaller size and the ease with the more complex weapons can be disassembled, conveyed in parts, and put together again, and can be employed to great effect. The Taliban didn't have many of those weapons and were still able to inflict considerable casualties. No reason not to believe ISIS can do better with more and better weapons. But, it won't happen.
The Real Deal
Hypotheticals aside, here is the situation there: A full-scale ISIS attack is highly unlikely and the current narrative is one largely manufactured by the media.
1. The media paints a picture that the Marines are alone and nearly surrounded and have already been attacked. This is bull-shit. Eight suicide bombers attempted to get into the base and were killed. Daesh has been ineffectively employing indirect fire at the base since October, which in itself is nothing remarkable at all. Happened every day at most bases during the war. Most of us didn't bother getting up from bed during an "attack." It is useful for counter-battery practice though.
2. I have been to Asad, I can tell you it is a big base (the size of a medium city) with a lot of ground and fortifications to overcome. To get to any Marines, Daesh would have to first overwhelm the IA that defend the base. The entire IA 7th Motorized Infantry Division is stationed there. They are a mixed light-medium vehicle unit with a lot of APCs and some tanks plus artillery. These are not Sunni formations (for the most part). They will not run from Daesh like the divisions in the north did. To overcome an entire division bent on fighting would take the entirety of ISIS manpower in Iraq. And even then, the fight would take weeks and IA reinforcements would crush them.
3. If the situation somehow became dire, like the 7th Division fleeing en masse, there would be more than enough time to evacuate the Marines.
4. The media has severely overstated the impact of the ISIS advance in Al-Baghdadi. They caught the IA by surprise captured parts of the town and executed people for the cameras. Their assault was contained and then they were pushed back out of the city. They never assaulted Al-Asad or even increased the volume of their indirect fire. This is a manufactured story.
