In order to do this, you need the Hypnotize Big Daddy Plasmid and the Natural Camouflage Tonic. All you have to do is use the Hypnotize Big Daddy Plasmid on a Big Daddy so that he thinks you're a Little Sister and protects you. Now here's the fun part - simply find and attack another Big Daddy and your Big Daddy will attack it to protect you.

About halfway through the game, you meet Sander Cohen, this crazy artist dude. When you finish the level, you can just leave him or kill him. If you kill him at that point, you can't get one of the Achievements "Find Cohen's Room." A while later you will stumble onto Cohen's apartment. If he's alive, two Splicers will be dancing in the main room, with Cohen watching from his locked private room. Kill the Splicers and then Cohen comes out to fight you, thus unlocking the door and allowing you to earn the Achievement.

When fighting a Big Daddy, use armor-piercing rounds, as they will deal considerable damage compared to regular rounds.

Use a blast of the Electric Shock Plasmid to temporarily disable security drones before you do anything else in a room. Then, run up to them within five seconds and hack them! You can hack while the game is on virtual "pause." You'll have security drones on your side for the rest of the time you are in a fight after doing this.

Once you get the Grenade Launcher and the Proximity Mines, this technique can be employed. Find a red fuel tank (all over the place), and cover it with about 6 Proximity Mines or so. Next, use Telekinesis to lift the massive explosive device and hurl it at a Big Daddy. This deals massive damage, leaving you with minimal effort to finish the job. Just be careful not to have the bomb blow up before you get there. (The launcher upgrade that deals no damage to you is a great thing to have just in case.)

While using the Telekinesis Plasmid, you can drop the item you are holding instead of throwing it across the room. To do this you must have an item currently being held. Pick up any item, with the Telekinesis Plasmid selected. Using the LT button, keep holding the item, and don’t let go of the LT button.

When you’re ready to drop the item, press the RB button to open your weapons menu, and in the transition of the weapons menu opening, let go of the LT button. Choose your weapon and the item will drop safely to your feet. If done correctly, the item falls at your feet, instead of being thrown, which makes the item easier to find when you need it.

Added by SyrionSyronimus: A simpler way to drop the item you're holding is to just hit 'X'. So, with the Telekinesis Plasmid armed, hold LT to TK something to you. While it's 'in your face', press X to drop it on the ground. No need to switch to weapons.

The following works with both the Crossbow and the Grenade Launcher. While shooting either, you can rapidly reload by pressing the left trigger, then quickly after pressing the right trigger. This will allow you to start shooting without having to wait longer (several seconds) for your weapon to reload. This is especially useful against powerful enemies like Big Daddies.