The lack of medical personnel is very important because it means those within the group are going without standard medical care and especially emergency medical care. I would wager that they are also not willing to go to a main stream hosipital to take care of major medical issues and this may actually lead to deaths from things that we would consider minor. As to how to exploit this weakness, well that is another thing all together.
I believe that ISIS is significantly weaker than they purport. Their hate is only attractive to uneducated or evil zealists who are incapable of higher reasoning. The men and women who participate in its activities are not as advanced as the men and women who stand against this great evil. One such discrepancy is in medical personnel. Opposing armies can attack through forcing ISIS into conditions prone to sickness and depend on less ability to treat wounded. This leads to a great advantage.
The overall lack of infrastructure present within ISIS, which certainly includes not having key medical personnel is certainly an aspect of the state that could threaten it's very existence. The inability to adequately treat wounded soldiers can be responsible for an increase in battlefield casualties as well as a drain on the overall morale of fighting forces, sapping their capability to hold existing gains in addition to capturing any additional territory.
While the lack of medical personnel is a legitimate weakness, it would be not ethical for opposing national powers to take advantage of that chink in the armor. The Islamic State does commit atrocities but it is important to keep in mind that it is still made up of people, including those who have had no part in the horrors it's known for.