How to beat the second boss?

[Anthelm]

Hi, that boss is difficult as hell, how did you guys do?

[ElDudorino]

You're talking about the huge spider in the frost biome?

Rule #1 is that you need space to move, and this applies to all bosses. Some bosses come with a pre-made arena where you already have that space, but the spider is just randomly anywhere in a cave. For that reason, before fighting you should clear a large space for yourself. You can do this over the course of 5 minutes with a pickaxe or 20 seconds with bombs. Since bombs are extremely common, I choose bombs. For this specific boss it actually doesn't hurt to make a couple medium-sized arenas instead of one large one so that if one gets covered in ooze, you can fight in the other one.

Make sure you have either sprinting boots like the zephyr boots or a dash like the force of wind dropped by the first boss. When the spider goes into the mode where he zooms towards you, you want to be able to move out of the way quickly.

For your weapon, you probably don't want something that has you standing right on top of the spider to hit it. So a starter sword isn't a great idea. Anything with a projectile is a good option, or a spear can work.

The bosses keep getting harder from here, so you can think of this one as training for the others.

[Anthelm]

You were right, i did it :) It's way easier when you open with bombs yeah

[phonon5891]

Arena and general strategy has been covered so I won't repeat that. If you're still having trouble, then another option is to overgear yourself -- the demonic pickaxe (which you should have by this point) can mine ores/minerals from higher-tier locations such as swamp (ivy ore) and desert caves (quartz), which you can then use to craft better gear. Should help make the fight much less dangerous.

Personally at this stage I like using the machine gun and quartz armor, with speed boosting or extra armor enchants/trinkets as necessary. Make use of the base summon slot (even non-summoners have at least 1 slot), free extra DPS can't hurt. Focus on avoiding poison pools and the boss' dashes, and keep your HP up with pots. When boss HP gets critical, it starts spamming attacks like there's no tomorrow, so at that point focus on bringing it down ASAP.