In most games, you can queue up attack commands by using the shift key. You can now do this with your pet so that it will kill the targets sequentially in the order you give it. This acts like a normal pet attack button, but when you hold shift and use it, your pet will attack your current target only once it is done killing all its already queued targets.

NOTE: Your pet will ignore the queue when he is in passive mode.

/focus [target=pettarget,modifier:shift]
/petfollow [nomodifier:shift]
/petattack /target [modifier:shift] focus
/petattack [modifier:shift]

There is also an alternative macro for this which activates the queue with a simple mouseover of the next target you wish your pet to attack. This saves you from changing between targets while shooting down another mob.

/focus [target=pettarget,modifier:shift]
/petfollow [nomodifier:shift]
/petattack [target=mouseover,modifier:shift]
/petattack /target [modifier:shift] focus
/petattack [modifier:shift]

Based on the macro above for Beastmaster. Behaves the same, Ctrl gives Bestial Wrath (no attack), Alt gives Intimidation (also no attack).

#showtooltip
/focus [target=pettarget,mod:shift]
/petfollow [nomod]
/petattack [target=mouseover,mod:shift][nomod]
/target [mod:shift] focus
/petattack [mod:shift]
/cast [mod:ctrl] Bestial Wrath
/cast [mod:alt] Intimidation