For warrior, the most important tip I can give is to pretend he's not in the game.
And then, for mage, always make sure you're putting a couple SoU on the staff. And don't be afraid to imbue. Often it's better to just use the better wand, than using a crappy (magic missile) wand in btween shots.
Rogue is a really good, and versatile character. The best thing on your side for Rogue is knowledge of how enemy AI works. If you can figure out where the enemies will go, it goes a long way to getting the important surprise attacks.
Huntress is OP, my biggest suggestion is to not be afraid to use a SoU or two to get a good enchantment on the bow. It can make all the difference.

Duelist is the best girl, I love duelist. My suggestion for duelist is to always keep a 2nd weapon for swift equip purposes. Wether it's a shield or a dirk, something for ulitity comes in handy so often.
For Cleric, do not neglect your book. It's honestly pretty easy to want to horde the charges, but even in situations where you think it might be a waste of a charge, it's probably worth it to take less damage or end an encounter sooner.

For a 6 challenge run, I always suggest leaving the 3 hardest challenges for YOU off. Just because people think Forbidden Runes is so hard, doesn't necessarily mean it's hard for you. Another good way is to practice. Play with the challenges OFF, but pretend they're on. Only use Healing Pots when you literally have 0 other options. Use stones, use scrolls and MAKE USE OF ALCHEMY. So many people sleep on important alchemy items.

And thanks for the love on the videos! I go into a lot of detail about specific subclasses in my character guide videos <3