Use Guards against single-target cards like Prince, Dark Prince, Mini P.E.K.K.A and P.E.K.K.A. Their shields keep them alive against small spells like Zap or The Log. On offense they distract single-target defenders next to a tank.
Tips
Spread them around a single-target troop to distract it for a long time.
Watch out for splash melee troops like Valkyrie or Bowler.
In Lava Hound decks they take care of ground defense.