-Weapons that give +power are nice to have. If you manage to level them up it's even better.
-Having a strong summoned creature makes your life much easyer. Sometimes they have strong cards, even equipment that you can cast on yourself.
-Some sort of "get off me" tool to escape will help your survivability. Control cards like "fear" or movement cards with [switch] will help you if you are surrounded. Placeable furniture will block pathing for enemies. Freeze is also great.
-Also, getting surrounded will get you killed. Don't get surrounded.
-Instant heals (+hp cards with "instant" keyword) are very good for defence, because they don't end your turn.
-If you think it's getting too hard/dangerous, don't be afraid to press esc -> escape. Escaping a dungeon will allow you to reset it, and grind some more cards/levels safely.
-If you think some card is overpowered, it probably is, don't be shy to cheese your way to victory.
-If something deals a lot of damage, you probably want it.

That's some generall advice off the top of my head.

im personally a big fan of powerbite; high damage for 1 mana.

Alter card by Rarity. Busted as hell.