Rom, the Vacuous Spider
The Byrgenwerth balcony overlooking the lake might seem like a dead end, but leap into the moon's reflection on the water to journey to strange dimension. What's waiting for you there? Rom, the Vacuous Spider.
Rom himself is a pushover, but he summons an army of smaller spiders to make trouble for you. The first thing you need to do is take these little pests out. Thing is, they're heavy skulls nullify most frontal damage - you'll have to sidestep around them and attack their abdomens to do any decent damage. Whittle down the mob until you have a clear shot at Rom, then lay down some heavy damage on his soft belly (like the minions, his skull is also defence-heavy).
After a moment, Rom will teleport to another side of the lake, but keep using the same strategy. Once you've done enough damage, he'll gain some actual attacks, so be on the lookout. The first is a barrage of crystals he'll fire at mid to long range. You can outrun or sidestep these, but the best defence is to get close enough that he won't use it. However, being in close puts you in danger of his other attack. When he rolls over, avoid any blue spots that appear on the ground - these will burst upwards with more crystals. Keep killing off the spider squad, and you'll have no problem taking Rom down.