Crypt of the Serpent King

Similar to previous Hard Levels, I lured all spiders I encountered back to the lava pits to kill them at a distance.

I had the Sword and basic Crossbow equipped. My stats (Strength/Endurance/Agility) were 1/6/10. I missed out on some XP in previous levels from enemies dying in pits and not killing any goblins in Level 2.

I decided to buy the basic Crossbow while entering Level 6 instead of saving for the final Longbow because I had used the bow to kill the Level 6 boss previously on Medium. Even with losing some potential gold by buying the 1st bow going into Level 6 and converting some gold in chests to arrows (and not picking up any gold in Level 2), I still had plenty of gold going into Level 7 to buy the final Longbow.

I would recommend using the Sword instead of the Crossbow to kill the spider across the gap, as I found the arrows are helpful for the final boss. I went into the final boss with about 37 arrows.

I killed 17 spiders across the lava and lost a small amount of XP from 4 spiders killing themselves in the pit. It took 23 minutes to clear the level.

I killed the Level 6 boss on Medium with the basic Crossbow, so I used the same strategy on Hard successfully. The boss activated more quickly this time, so I was unable to get an initial arrow hit before he started moving.

I once again used a constant wide clockwise circle strafe by holding the Left analog stick fixed in the 7-8 o'clock position while gently tapping the Right analog stick to the 3 o'clock position to try to aim the reticle over the boss, turning it red while continuing to spin.

The boss more-or-less attacks constantly, so I didn't bother to time my attacks. I tried to adjust the reticle when I could and fired arrows continuously, landing about 2/3 of them. I didn't get hit, but it seems you might be vulnerable whenever you fire, as your character seems to pause even when still holding the Left analog stick.
It took roughly 8-12 Crossbow arrow hits to down the boss.

For this Level, just keep doing what you've been doing to get here. By now, you should have Agility and Endurance turned up as high as possible (probably maxed out, and maybe a couple points in Strength, if you've been killing lots of monsters). Save up for the sword as early as you can, as it's the best weapon, but on Hard, I actually had better luck getting each melee weapon in turn, one after another, in order of price. But you should buy the most expensive weapon you can afford, between levels. Either way, you should have enough gold for the sword, by now. I also bought the bow (not the crossbow) at this point, but I didn't find it very useful. If you have enough arrows, it can keep you away from the snakes in Level 7, but I found the practice of strafe-melee more beneficial.

To fight the enemies, you can walk near them or let them get close to you, then back up as soon as they start to strike. As soon as you've dodged the strike, attack and move forward to land a blow just before they attack again. Rinse and repeat. Sometimes, you can get in enough strikes to kill them after their first attack, but it can be risky. Sometimes they use a quick attack that can squeeze in between your own attacks, so it's probably better to play it safe (since there are no health refills on Hard).

It kind of works on the boss, too, but I found it easier to strafe around bosses and swing your weapon when the crosshairs would line up (the crosshairs turn red when you are in range and lined up, but you have to time it with the actual impact of your weapon). In fact, on hard, I found it to be good practice to use this method on most of the enemies. If you are strafing in a circle around them, they can't seem to hit you (they swing, and then you've strafed out of the way before impact). If you have room and you can get proficient at this method, you can avoid taking damage at all!

If you need more detailed help, check out the Solution for beating level 7 on Hard: