Machiavelli didn't like mercenaries, something that is evident both from The Prince and (I see from a quick glance) The Discourses, where he writes that good soldiers are the sinews of war, not gold, and that it "is as impossible for good soldiers to fail to find gold as it is for gold to find good soldiers" (Penguin ed., 1970, p.303).
In chapter 13 of The Prince, Machiavelli continues a lecture against the use of mercenaries begun in the immediately preceding chapters. Among other things he criticizes King Louis XI of France (reigned 1461-1483) for his reliance on Swiss pikemen:
...when he [Louis XI] gave reputation to the Swiss, he debased all his own arms.... For after they [the French] had become accustomed to fighting with Swiss, they did not think they could win without them. From this it follows that French are not enough against Swiss and without Swiss do not try against anyone else. Thus, the armies of France have been mixed, part mercenary and part their own. These arms all together are much better than simple auxiliary or simple mercenary arms, but much inferior to one's own. (pp.56-7)In referring to French weakness, Machiavelli was thinking of a then-recent event: the French "having been forced out of Italy in 1512" (according to an editor's note in the Skinner & Price edition of The Prince, Cambridge U.P., p.50). However, Machiavelli does not mention that Louis XI largely owed the Swiss his victory over Charles the Bold of Burgundy at the battle of Nancy in 1477, a battle that altered the geopolitical landscape of western Europe. [Correction/clarification: It was not really "his" (i.e., Louis XI's) victory; see the discussion in the comment thread.] Machiavelli says that "a wise prince...has preferred to lose with his own [arms] than to win with others, since he judges it no true victory that is acquired with alien arms" (Mansfield trans., p.55). Louis XI presumably would have begged to differ.
Postscript: The problem for the Swiss mercenaries, according to Michael Howard (in War in European History, p.28), was their slowness to adapt to changing conditions of war: "...as shot became increasingly important and formations increasingly flexible the Swiss pike phalanxes became left behind like dinosaurs unable to adapt to a new environment, as much of a curiosity in the history of infantry as the English bowman of the later Middle Ages."
Btw, what would Machiavelli have said about how to combat ISIS, or about any other current issues? Is this question even worth asking?