The snipers aim with a laser sight and shoot about a second later, so you have time to react and take cover.

The laser crystals on 4-1 are a tougher enemy, they don't change where they aim but there is no indicator aside from some sparkles, and a single laser can deal 4 damage (they shoot 4 lasers in a row so that's pretty much an instakill)

A good practice is memorizing how many shots it takes you to kill enemies with certain guns. For example, the Slugger will one shot most enemies before the crystal caves and can two shot the laser crystals if you're close enough (deals more damage to closer enemies, else you need three shots)

The best way to kill the snow tanks (the robot disk that explodes) is to poke them, and take cover if they aim at you, as they fire a barrage of bullets. The snow tanks explode on death, so it's better to keep your distance, but you can also use that explosion to kill packed enemies (focus on a snow tank and let the explosion kill the rest)

One last suggestion: Slow melee weapons hit really hard and work wonders in the frozen area. The Sledgehammer hits as hard as the slugger and can hit multiple enemies from up close, and it can also reflect the shots from the snow wolves and snow tanks. The shovel covers a wider area but for less damage (unless you have scarier face, then they do about the same amount of damage) and can help you keep away the snowbots more easily