Well first off I plan on making this the most in depth, impressive guide ever so I will explain every level and loads of strategies.
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On Level 1 we start out with our good friend the USP Pistol. The goal is simple, eliminate the Brown Seals with two headshots as this will give you 20 more $ and finish them off quicker. No one will take you seriously in Seal Hunter if you can't get at least 90% headshots on Brown Seals. Now clearly this is not as easy as it seems for new players, the sliding on ice will get some getting used to. What works best for me, as I'm sure is the best method, is to run to your destination and then stop yourself with the opposite keys; so essentially if you want to run up, run up and once you reach your destination then simple tap the back key so you stop. Once you've mastered this you can get the maximum number of points you need on Level 1.. or can you? You can actually obtain more points in Seal Hunter by overkilling the enemy, this can be done by using grenades or the pistol-magnum combo on Level 1. The pistol-magnum combo is easy to do, all one needs to do is shoot a seal in the head with his pistol, then switch to his magnum and finish them off with a headshot. Done correctly, the seal will explode and you will be rewarded with an extra 15 points. But wait, you've decided to use the MP5, a wise choice considering the MP5 is arguably overpowered and can get you to Level 4 without a problem. Although the MP5 is fully automatic, it does not excuse you from getting headshots; it's actually rather easy to get headshots with too once I explain the accuracy of the gun. The MP5 shoots in three places; middle, above middle, and below middle. If you aim slightly above the head of a seal then you can hit the Seal in the head with both the middle and below middle shots while the above middle shot flies off in the distance. If you aim at their head however; the below middle shot will hit them in the body, causing you to lose a couple of points. Now let us go to the grenades. The grenades are an odd weapon in Seal Hunter as it is both underpowered and overpowered. When enemies are beyond the middle way point then it is a rubbish weapon, when they are in front of it however, it is a weapon to be feared (it also overkills the enemy, granting you more points). Overall I'd say the grenade shouldn't be touched unless playing a 3-4 player game; 2 if you're rather daring. The grenades are very tricky to use and unfortunately it's really up to you if you want to be good with them because all I really can explain is that if you go forward and throw one then it will go farther and if you go backwards and throw one it will land closer next to you - and that's really not helpful. With this weapon, practise makes perfect. The boss on this level is the Fat Seal, and he is quite easy. He regains his health when he stands still so no use hitting him when he's like that. Just wait till he starts moving and then shoot him (in the eye if possible as it grants a 1.25x multiplier). It is possible to kill the Fat Seal with only the beginning pistol. Once you kill that boss (which is so easy I don't think deserves more explanation) you're off to Level 2. Let me explain the weapons a little more though.
If you have a pistol at this time, it is time to get a weapon. Preferably something less or equally expensive as a Shotgun.
If you have a Magnum, stick with it until you are ready for a Shotgun (if you plan on taking out the Polar Bears with this) OR an M16 or higher
If you have a MP5, same thing as above.
If you have a grenade, same thing as above.
The weapons I use in 1 player are MP5 -> Carl Gustav -> AWP; which I will explain later.
In two player I use MP5/Magnum -> Carl Gustav/AWP/Minigun (depending on the circumstance and if my teammate has lots of money)
In three player I use the same thing as in two player.
In four player I use the same thing as in two player.
The MP5 is always your best bet because it's easy, efficient, consistent, and costs hardly anything - as I said before, you can easily make it to Polar Bears (sometimes even past Polar Bears (in 2 player +)) with this weapon.
*Level 2 - Level 5 to come later