Get a fully upgraded boosting partner, and set the difficulty to immortal or higher. Immortal is good enough to obtain gold. Both of you should just rush through as quickly as possible, without getting hit once by any enemy, and have the fully upgraded character/class do all the work. The weaker character/class should just block and dash, for the most part. Try to find any quick to get to Secret Areas, as well, as these give more points the harder the difficulty is.
Alternatively, this can be done alone, on immortal, if you're just starting a new character and all of it's equipment and stats are low. Everything is almost the same stated above. Rush through the level as quickly as possible, but if you have to kill enemies to advance: for all your characters, take advantage of the stun abilities and kill the enemies without getting hit to max your multiplier and get more points. But when the enemies regain their consciousness, be sure to dash away to avoid getting hit and lowering your multiplier. Rinse and repeat. Explosive barrels are a big plus to your multiplier, especially when enemies come at you in numbers. Take advantage of that as well. Don't forget to check for secret areas as well. If it proves too hard, gather up enough gold to buy better weapons and armor and try again.