The sniper rifle becomes available later in the campaign, but it's one of the most useful weapons in the game, particularly in the mission where you find it. Use the sniper rifle at extremely long ranges. Use its zoom mode (press the alternate fire button, then use the mousewheel to zoom in closer) to pinpoint the location of enemies. Hold the crosshairs steady and fire the sniper rifle, ideally at the enemy's head to score an instant kill (otherwise it may take two or three shots depending on the enemy). Conserve sniper ammunition carefully. Eliminating enemies from the safety of long range is an important tool. You save ammunition from other weapons, and you aren't at risk of sustaining damage, which would then require a hunt for health and shield energy.
Isaak builds the spidergun from elements discovered during the battle with the arachnids. Use its primary fire mode to engulf the enemy with small arachnids (hit the enemy, and he's covered with the small spiders). The alternate fire mode drops a pile of spider eggs on the surface; the eggs hatch when an enemy stands nearby, and the spiders wreak havoc on the unsuspecting enemy. Hold down alternate fire to create larger eggs that release larger arachnids. While the enemy is busy against those spiders, cycle to another weapon and blast him.
Drakk Laser Rifle
This laser rifle is a high-damage, accurate weapon, but it possesses a slow rate of fire. Therefore, maximize each shot by lining up your target carefully. If you miss, the enemy has plenty of time to retaliate while you wait for the rifle to reload. The drakk laser rifle works well against its namesake--blast the drakk with their own weapon. Tougher drakk drop rounds of the rifle's ammunition. Isaak eventually tweaks the laser rifle to add an alternate fire mode. Hold down the button to fire a steady laser stream. Point it at your target to inflict damage as long as it remains on the target.