Follow the green arrow when you get as it shows the path to your next objective.
Don't go opening doors at random rather try exploring the level to spot which ones should be unlocked first.
Always try to pick enemies off at a distance. It usually fatal etting them get too close.
When you have a taskk of destroying a reactor, you must make sure that you've collected all of the keys across a map before taking the reactor out. Destroying it will start off a timer that will destroy the facility and if you haven't reached the exit by that point (lack of keys), you can easilly start a mission, again.
A black hole on the floor!? Aliens will spawn from them as long as you're in the vicinity. Get in, grab everything and get out of there as quickly as you can.
Difference between each of the two modes (classic and enhanced) is that the classic, original version features a single stick for both shooting and firing (tricky), while the enhanced edition takes advantage of double virtual sticks. For the touchscreen, we feel the twin-sticks are a no-brainer.