To earn 21 stars you will have to earn 3 stars on all 7 challenges. Recommend to play these challenges in co-op (can be done solo), but in co-op if a player gets down there will be a 3 second spawn wait, which is better than having only 1 life playing in solo.
Money Shots - Gather (Possible solo, better in co-op)
This challenge will require you to take 10 dirty pictures within a time frame. Play this challenge in co-op, its much easier. From the start have one play go right, while the other player goes left. If you or your partner are having trouble, run over to them and help keep the area clear. Recommend using Meg and switching to her secondary weapon.
Amish Amuck - Elimination (Possible solo, better in co-op)
By far the easiest of all the challenges. In this challenge you will have to kill 50 Amish and killing less than 3 tourist. Killing more than 3 tourist will cause the challenge to end. Playing in co-op will make this go by faster. Recommend using Brains shotgun or Lois needler weapon. Try to aim low and not kill or hit more than 3 tourist.
The Crips and Ables - Rescue (Possible solo)
In this challenge you will have to free 6 hostages from the alien prison within a very generous time frame. Recommend Quagmire using his assault rifle and going for head shots. Watch out the handicaps can beat you down pretty easily, so be on the look out for health packs.
Town Hall Beating - Wave Defense (Possible solo, Better in co-op)
In this challenge you will be going against 3 waves of enemies. Again, playing on co-op, if a player goes down there will be a 3 second spawn delay, and is recommended.
You will be playing as Adam West. Try and stay at the top of the stairs and use Adam Wests dual magnums to take out close enemies and the sniper rifle to take out the further enemies (sniper rifle is highly effective against the robot mechs). Using special inventory items, regularly, and keep health packs with you will help tremendously.
Again, playing on co-op, if a player goes down there will be a 3 second spawn delay, and is recommended.
Sea Men Attack - Wave Defense (Possible solo, Better in co-op)
In this challenge you will have to survive for 6 minutes, without your pirate ship being destroyed. Again, playing on co-op, if a player goes down there will be a 3 second spawn delay, and is recommended.
You will want to focus on using the cannons. As the regular weapons and special items wont be much help. They will only come to help to keep the pirates coming aboard or when they are aboard. Both players shouldALWAYS be manning the cannons. If pirates do come aboard, have one player using the cannons and the other clearing out the pirates on board.
Last Minute Shoppers - Escape (Possible solo, better in co-op)
In this challenge you will be running through Santa's Workshop from the opposite direction of story, while collecting 5 bombs and reaching the end with 15 seconds on the clock. The enemies aren't a treat, so you can easily keep running. There are an abundance of health kits to keep your health up.
If playing solo, when you pick up the first bomb, run back to where you spawned and jump off the railing, to make traversing the spinning gears much easier.
If playing co-op, you wont have to worry about starting over if you fail by jumping over the railing and also you will have some back up fire taking out and treat while you collect the bombs.
Someone to Overwatch Me - Overwatch (Needs to be done in co-op).
This challenge requires you to do it in co-op. One player will be in control of Peter flying a helicopter. And the second player will be in control of Lois who must plant a series of eight C4. LOIS MUST NOT DIE, if so, it will be game over.
-Player one, Peter, will be defending Lois. Shoot down and snipers in the building with the helicopter mini gun, (she will die fast) so don't be afraid to spray. The mini-gun has infinite ammo, but you still have to reload.
-Player two, Lois must make it to the end of the level by planting the eight C4 and defend herself from any ground enemies, which shouldn't be a problem using Lois's needler gun. Make use of the structure columns to take cover from the snipers.