For instance, take wounds to avoid wasting time. (See below.)

I didn't realize this at first. I avoided taking damage by spending more time. This really hurt me, because there are five different things you need to accumulate if you want to fight and beat the boss, and health isn't really one of them. The boss can sometimes kill you in one round, even if you have as high as 13 HP going in. You need potion tokens, potions learned, skills, and items. To get those, you need more time. HP is important, but unless you're going to die, saving time is the way to go.

Your pick of hero can really make or break the game, and can strongly influence your strategy and decisions. When I first started playing campaign mode, my hero was the Archer because she seemed the most balance in terms of how many dice of each color she started with. That was something of an error, because while her dice were balanced, her Heroic feat (spend 2 time to roll 2 and then discard 1 heroic die, or spend 4 time to roll 3 and then discard 1 heroic die) really burned through the deck. Combined with starting in the Dragon's Lair, and I really struggled to acquire enough items, experience, potions, tokens, and skills to make it to and defeat the dragon itself. The Archer isn't bad, it's just hard. Choose your campaign hero wisely.

If you're playing single-player, you may be tempted to "knock over" a 1 and turn it into a 3. Don't fall into this trap. Cheating only makes the reward feel lesser. It makes you more impatient, self-doubting, and gives you a fleeting and false sense of accomplishment. Going through and legitimately winning, legitimately gaining campaign abilities, really is what makes the game fun and worthwhile. You'll just ruin the experience if you are constantly cheating.

Get some blank cards, extra tokens, something to mark some of those extra abilities. It can be difficult to remember you have this power from your campaign sheet, or that you get an extra health or potion, or that your Basic Skill doesn't count against your number of skills owned since it looks just like a regular skill card once it is under your character card. Little reminders of some kind can be a big help.

There are lots of smaller rules that may fall by the wayside on accident. All the time, I see people forgetting about (arguably) "smaller" rules, such as having to cover all visible boxes on the dungeon card, not just that floor's, or that you get a new potion token both when you learn a new potion and when you level up. Little rules can have big impacts on your strategy, the difficulty, and the enjoyment of the game. Rules are there for a reason. If your homebrewing rules and purposefully changing or ignoring them, that's fine. I encourage creativity! Just try not to forget about the smaller rules. Once you beat a difficult encounter and move on and then realize you forgot to fill in all the dungeon's challenge boxes, well... it's a bit like a cold cup of water being thrown on you!

I love that you can play ODD alone, and I've played campaign both as 1p and as 2p. For me, campaign was more enjoyable as a 2p. We even rollplayed some, coming up with character names and vague backgrounds. Instead of just trying to win, having a second player to talk strategy and cooperate with, along with a heaping side dish of banter and teasing and funny fake accents, really made the game 10x more fun. It might not be the case for you, and it isn't the case for all, but I'm a big fan of trying new things, and some people I know have been pleasantly surprised by how much they enjoyed 2p campaign ODD.

This is pretty self-explanatory. Don't give up just because you've hit a slump or a difficult part, but if you find yourself continuously miserable playing, don't play. Remember this is just a game, and if you win or loss, it's supposed to be fun. It doesn't reflect poorly on you to loose. It does reflect poorly if you keep punishing yourself and making yourself repeatedly unhappy and stressed in order to have the bragging rights to say you beat a card game.