Poor Old MacDonald. I really feel for the guy. All he has left is a few pets and a dream. That still doesn't stop me from beating him once a day for the large amounts of lovely, lovely XP he gives!
Clucks is level 3. Your pets should all be level three after soundly beating up the snake lady. So it's all good. She's also a Flying pet (even though she's a chicken). So that means she takes less damage from Beast abilities and does extra damage to Aquatic pets. It also means she takes extra damage from Magic abilities. So if you've got a Beast (and you should), put it on a different pet. And if you've got a Magic pet. Sick it on her!
Clucks also does this three round ability called Flock. This is where you get run over by a flock of angry chickens. They don't do much damage individually, but they put a debuff on you for two turns that causes you to take 100% extra damage. This is not good. You will see this kind of ability many many times when fighting tamers. So remember, if your pet has medium to low health (or even if it doesn't), you should consider swapping it for another pet after Flock is done because otherwise it may take two very large hits.
Foe Reaper 800 is a tiny mechanical harvester of death! He harvests corn and okra too, but mostly he just harvests death!! Actually, he's not that tough. However, since he's a mechanical he does extra damage to beasts and takes less damage from magic. This means if you have a Beast or Magic pet you really want to send it against someone else. Also he takes extra damage from elementals, so if you have an elemental here—attack!
Teensy is a mouse. He's sort of cute and your Beast pet will find him very delicious. Also, keep any Undead pets out of reach of Teensy. That is all.
In conclusion: choose the right pet order and this should be an easy victory.