When you enter an area and hear a growl...backup and prepare to fight!
Look for unlikely or seemingly impossible shot angles. By this I mean it can appear that you are shooting over the heads of the target but you are actually making hits. There are numerous situations where you are firing down on your target, aim high and you'll still likely hit your target. The level where the scorpion-like creatures comes to mind. In one situation there are several that can't get to you because they are behind short barricades. All you can see are the tails. I found that by backing up a bit and aiming upwards that I could hit and kill them even though I really did not have a clean shot.
Keep in mind that you can buy the necessary keys if you cannot find one or one is too difficult to get.
Remember...almost all of the seemingly impossible situations have a simple solutions so just experiment.
One technique that must be mastered is movement from side to side. This allows you to aim/fire the weapon and dodge shots coming at you at the same time. Rotating and firing the weapon is not a good idea unless the attacker is right on top of you. My personal favorite control configuration is to use a joystick and set the control menu option to mouse. This seems a bit odd but what it does is allow rotational movement in two ways, the mouse and left/right joystick. Side to side movement is now the left/right cursor keys and forward/backward is the up/down cursor keys. Forward/backward is also the accomplished via the joystick as well. Even though the control is set to mouse, I really don't use it much, mostly just to make minor aiming adjustments or to adjust my course slightly when walking down a hallway.
It's good technique to put some distance between yourself and your opponents. This gives you time to dodge oncoming shots. Firing while backing up (also using side to side movement) is very effective. For this I will take my right hand off the joystick and utilize the cursor keys while blasting away with the left hand on the joystick fire button. If an opponent gets too near then I switch my right hand back to the joystick so I can rotate and rid myself of the attacker(s).
Another good technique is to walk part way into a room or hallway and then back up. This will often cause the enemy to appear.
Boost the simple shot as soon as possible. This can occur as early as the first world, second arena. Obtain the fireballs as soon as you can. This will probably happen in the first world, fourth arena. Boost this as soon as possible, probably the very next arena. Stay away from the flamethrower, I think it's a waste of credits. The plasma gun is powerful but buying it will delay the purchase of more powerful weapons.
After boosting the fireballs you have a choice to make, go for either the magnetic gun or the death machine.
What weapons will win the game for you? Two have worked for me, the boosted magnetic gun and the boosted death machine. An unboosted death machine would probably work. An unboosted magnetic gun might possibly work but I don't think so. The magetic gun is far cheaper than the death machine so it can be obtained much earlier. Trying for the death machine means struggling along with boosted fireballs until the third world, fourth arena at the earliest. This is very challenging but once you get the death machine, look out! The rest of the game becomes fairly easy. Whichever weapon you choose, boost it as soon as you can.
For elevators going up you can sometimes see attackers and kill them by aiming up and firing. Sometimes no attackers appear until the elevator gets to its destination. In at least one situation, starting up the elevator and then backing off of it will bring on the enemy so they can be killed by firing upwards.
For elevators going down try firing over the edge to kill anything that might be waiting for you at the bottom. Another technique is to step onto the elevator and then step back quickly so that the elevator goes down without you. This sometimes exposes the enemy who can be killed now by firing downward.
At first, you will probably be spending lots of credits just to stay alive. This will prevent you from buying the necessary weapons that you need to take you further. As you become more skilled, though, this will taper off.
Once you become adept using the fighting techniques, you will for the most part only need to spend credits for weapons and for boosting them. Only occasionally will you need to spend credits for health and shields. Once you have obtained the more powerful weapons you may often need to spend credits for energy however.
Your numbers (health, shield, energy) at the end of one level become your starting point for the next level. If you have low numbers then you might want to replay that level again if you think you can do it better. Try to carry the best numbers possible from level to level.
1.1 - ------- N/A ------ | 3.1 - "181C7ETKOT5RQGPT"
1.2 - "18WSGGJKL13JSE2W" | 3.2 - "181C199KH5RRQGPW"
1.3 - "1KOSDHGOL7HJSEQV" | 3.3 - "183CRLVWJ7BRQGPV"
1.4 - "17DCDRVQMWHJSEQQ" | 3.4 - "18MSSPU4KWBRQGPQ"
1.5 - "18LC3C9SL4THSDQP" | 3.5 - "18NCNJBKO3BRWGPP"
2.1 - "18LS2UBGMJ1HSGPD" | 4.1 - "17BSHBWCKPNRWGSD"
2.2 - "181C24FOO1PHSGPG" | 4.2 - "18WC4HBQVQJRWGSG"
2.3 - "17GS7WQMIJ5HSGPF" | 4.3 - "178CJRDIUG6RWGSF"
2.4 - "181CTF9EKUPHSGPA" | 4.4 - "8BISLRSSR9URWGSA"
2.5 - "181CW6CMUW9HSGP9" | 4.5 - "8GJBPB548STRWGS9"
1.1 - ------- N/A ------ | 3.1 - "181A59KMV63PGOPT"
1.2 - "181CEICWL13PGOQW" | 3.2 - "181A59KOV63PGOPW"
1.3 - "181C8VWGMQ3PGOQV" | 3.3 - "181A59M2W6RPGOPV"
1.4 - "181CUBW2NN3PGOQQ" | 3.4 - "181A59OWPFBPGOPQ"
1.5 - "181A59KMOMVPGOQP" | 3.5 - "181A59IW5CBPGOPP"
2.1 - "181A59NMH5RPGOPD" | 4.1 - "181A59I21TDPGOSD"
2.2 - "181A59JSIURPGOPG" | 4.2 - "181A59L44U9PGOSG"
2.3 - "181A59KCKPNPGOPF" | 4.3 - "181A59NAF5DPGOSF"
2.4 - "181A59M6TSFPGOPA" | 4.4 - "181A59NCF5DPGOSA"
2.5 - "181A59M8TSFPGOP9" | 4.5 - "181A59H6G85PGOS9"