Please see the image. It should be most difficult to design drugs to combat animals and, after that, fungi.
Work Step by Step
The tree can be obtained from the olive box on p. 604, replacing the three ameoboid branches with the Amoebozoans as a group. The question of drug design difficulty involves phylogenetic closeness— the most difficult design should be for the evolutionarily closest/most similar relative, since such a drug would be more likely to affect humans as well.