This secret achievement is for beating all the bonus Protector Trials, as mentioned above by XXXICenturyMatt.
The bonus trials are unlocked once all the regular trials have been completed. There are 6 bonus trials (1 for each level), and you get to use all your guns and plasmids.
My recommended strategy for each level is to get the money to buy mini-turrets as quickly as you can and place 4 around the little sister. This will keep her well protected. I used the Insect Swarm plasmid constantly, which really slowed down the splicers. With those defenses and your favorite gun, protecting the sister (and even getting an A/A+ rank) should be no problem.
Use whatever weapon/plasmid/tonic combo works best for you!
** Reminder from Fire Hawk D : You do get to choose which tonics you want to use, and all of your plasmids are at level 3. **