Tips & Tricks (Ecrazeus Castle)

Here's a few tips on playing the game.

Basics to win

Collect power-ups as much as possible.
Remain in the Fever state as much as possible to deal significantly more damage.
Use items to heal when low, and consider offensive items when the situation might call for it (like arenas)
Collect new weapons as they're technically stronger than older ones.
Gain as much time as possible during the first phase of a course so that you have more time to gain points, power-ups, weapons, and items.
Room contents are limited: don't try to explore everything too in-depth.
Weapon enhancement levels ("Weapon Name +1", +4, +33, etc) will cap if the enemy level isn't high enough, so consider exploring more rooms to increase this cap.
However, when time runs out, try to clear out the room for any additional weapons, items or power-ups. There's no punishment for taking too long after the timer's gone.
Sell unused items and weapons to make money, and buy new weapons and needed items. New weapons will also benefit from the enhancement upgrades, just like a dropped weapon.
As long as your HP is above 1, you will survive. However, be careful, some of the invincibility frames caused by some enemy attacks are very short and they'll easily combo you if you're not careful. These are not standardized.

Scoring

There are ways to increase your score that might not seem obvious at first.

Every enemy defeated increases the score multiplier by 1. The score granted by foes is multiplied by this multiplier.
The enemy level increases the amount of base score the enemy grants. The enemy level goes up every time you go to a new stage/room, and can be seen on the UI.
When you defeat a foe, your two weapons enhancement levels (+4, +22, +7, etc) are added to the base score of the enemy kill before the multiplier kicks in. Equip two strong enhancement weapons as much as possible and don't rely on old weapons for too long (they're also weaker to some degree).
Boss kills are affected by the score multiplier and have a naturally higher base score compared to normal enemies.
Pickups are negligible score-wise in the grand scheme, but they do grant points.
If you stomp a foe while you're shrunk, you'll gain +1 to the multiplier, once per foe. Bosses give you more multiplier depending on the course, with the Expert course boss granting +7 to the multiplier, the highest amount.
By extending the timer as much as possible, you'll have more time to gain score, more time to increase the enemy level, more time to gain new weapons.
The end of course dungeon has a starting enemy level. If the current enemy level is higher than this starting level, it will go higher. Consider increasing the dungeon's level by proxy, but be careful of the difficulty.

However, there are also ways to make your score worse.

If you use a Continue, your score gets halved, and the multiplier resets to 1. If you have a good score, it might be worth giving up to preserve the score.
Using items will be the biggest contributing factor to your score multiplier being less than expected, especially if you're late into a run and require more healing. Use them as sparingly as possible. The amount it reduces the multiplier varies and is listed on the item description.
Most heal-on-hit weapons reset your score multiplier down to 1 per hit. The only exception is the Demon Shield. Do not use these if you intend to get a high score.

Secrets

For the Jump Girl achievement, consider using Faune, as she has the strongest stomp power in the game. You cannot ever equip a weapon once (the weapons UI should be completely empty). It's a very doable achievement, especially in Beginner course (although you'll need to figure out the alternate path instead of the barred gate, as you can't use weapons to open it anymore).
For the Beauty and Vista achievement, you'll need a special room event and some Mulkmont on hand.
For the Queen of Speed achievement, consider unlocking the "Show Run Timer" option and enabling it before a run starts. While it might be tempting to rush through the first phase, consider doing the opposite and rush through the second phase, as you'll be much stronger to handle it and the boss. Do not worry about the Wizamart shop, it should pause the timer in the shop menus.
For the Gamejam Nostalgia achievement, dress up like the protagonist in this gamejam entry and play on Advanced course, until you randomly end up in a school being rebuilt.
For High-Stakes Shielding, do not use it against a lot of foes as they might be unpredictable and lead to an early death: consider the humble Reaper, who has a very predictable slash attack when up close. Do NOT try this against the mage enemies, you WILL look stupid (personal experience, responsible for most of my failed attempts).
For Censorship Tried, it is in the very first room of Beginner course. Look for a strangely close area outside of the usual level area. You might need to jump on top of a pillar to reach it.
For the Hardest Quest, the requirements to spawn both EX Bosses are the same as Saving Myami and Buying Ecrazeus, but you must fulfill both conditions in a single run. It won't be easy sailing, but I have faith in you!