How to help a combative diabetic adult to administer glucagen (glucagon)?

Very carefully! Diabetic patients with severe hypoglycemia can be a challenge because they often won't agree with you what the best course of treatment is. As a rule, aim to give awake/conscious diabetic patients oral glucose (in any form - juice, pop, glucose gel, glucose tablets, candy, etc.). It will be easier to convince them to take that than a shot! save the glucagon injection for unconscious patients.