Use the battery to freeze them, then begin using the chainsaw by tapping on them until you can remove them with the forceps. If they begin moving early, use the battery again.
Partners
Only use your partners when absolutely necessary. The adrenaline shots should be sufficient to keep your patients alive during the first two chapters. If a partner is used, you must wait thirty minutes for them to become available again. Start using them from the Chapter 3 onward, but never spend coins to speed up their use.
Time management
To finish a surgery as fast as possible, you must optimize your time. Start with the worst injuries because they will drain the most health from your patient. Then, use the same item on all similar injuries. You will get a much better time and also increase the combo meter faster.
Tools
Although most of the time you have to work quickly, at certain points it is better to slow down briefly. For example, when using the chainsaw proceed slowly and steadily, either by using circular motions or by cutting the sides of an organ or object. Also, work slowly with the battery zapper and wait for the creatures to land. The green adrenaline shot should also be administered slowly for best effect. Use the forceps after cutting with the chainsaw to remove the item that was severed. When you have poison inside your patient's organs, use the red needle. Drag it very fast to get the maximum effect.
Upgrades
There are five upgrades that can be purchased with your coins. The most useful is the green adrenaline shot to give your patients more heath. Following that, purchase the combo bonuses for higher scores and finally the cutting object upgrades.