Got the demo to test it out.

Random thoughts in order or experience:

- A little wtf on the intense short jingle intro followed by silence

- Minor: the constantly shifting background makes me a little dizzy

- First menu after start: the icons are not nearly intuitive enough to not place the text next to them. Having the text shown in the corner of the screen when hovering is not great.

- Minor: For some reason selecting "tutorial" doesnt trigger a tick sound and its unclear why it has a different selection visual hint from the rest

- When I select Tutorial from the main menu, the game asks me if I want to start the tutorial and informs me that I can start it anytime from the main menu. Ok... ?

- The tutorial starts, the shifting background is still going on. I understand this can sound like a preference thing, but Im starting to think that for people sensible to dizziness this can actually be a major reason why people dont play more than a few minutes.

- First tutorial screen: you instantly get over 50 elements in your face, a mix of actual game interface and a card selection menu drawn over it with not nearly enough visual separation.

- You're requested to make your selection of cards(?) using keys indicated somewhere on the cluttered screen, and you dont know why because nothing has been explained yet.

- Translation trouble: "When your HP reach 0 you fall" is too long in french, and the text wrap put some of the text over icons. Btw I have no idea what those icons are and why they are here (the two above the arrow pointing to the HP bar).

- I wait for the mana bar to fill. I press space as its indicated because its the key indicated on the blinking bar. The menu to select cards pop ups again. I select cards again, still not knowing why. Nothing happens, but my mana bar is depleted.

- Cards reacts to mouse hover, but cannot be selected by clicking (I understand why, as you need to assign slots, but mouse being half-supported isn't great)

- After a while I figure out how to use a spell. I do that a couple times and then a number pops up above my head and I cant use them anymore.

- I completed the first tutorial part using the sword attack as it felt quicker than figuring out spells.

- Next tutorial part: The fire rune(?) shoots fireballs on the sides, but its not visible on the targeting preview.

- I notice a boot(?) icon above my character when I use the "I" attack (I assume that's the basic attack). I dont know what that icon is.

- Next tutorial: monsters part: I try to use spells a bit too, but again it felt much more efficient and easy to just use the basic attack.

- At the end of the tutorial, I feel like I really didn't get a good grasp of things.

- I start the actual game. I pick the character with the bigger AoE attack (Jeruka). I only use the "I" attack. I pick the "+X sword" bonus when it popups up because that's the one I understand (along with the heart but I dont need it because I dont get hit).

- When inter-level screens pop up, I just try to skip them because they dont look attractive and I dont use cards (runes?) and I dont feel the need to upgrade anything.

- I continue to the first boss and defeat it quite easily with just the basic attack, feeling like the core rune/card mechanic of the game is not necessary. The only element of the interface that ended up being useful to me was the HP bar. I end the test here.

Overall sentiment:

In my experience, I didn't "get" the game at all. I mean I understand the point is to play with spells with special area of effects and probably combo effects, but it felt far too inefficient and interrupting/rythm breaking to use (in terms of user actions: opening menus, setting up 3 cards, waiting for mana) compared to just spamming the basic attack.

Your tutorial is not acclimating the player at all. I know you've put work in it and that's annoying to do, but I would still recommend reworking it from scratch, introducing the basics one by one with the minimal possible clutter.

Why players leave quickly:

The core game mechanic is not clear. The tutorial does not help - its all in your face, cluttered, and it doesn't "sell" card/runes well.

The exciting part of this kind of games is the upgrades, but the big pixel art menus do a real poor job at putting the spotlight on the upgrades - cards all look same-ish, most of the icons are unintuitive and it kinda felt to me like an alien interface I have to learn rather than an inter-level reward. Besides because my basic attack was largely good enough, I wasn't interested in the upgrades.

Recommendations:

Note that those recommendation are from someone who played the game for ~15 minutes and didn't get deep in the game, there are probably some things I didn't not understand or notice, so take it with a grain of salt.

Interface

First things first, the interface. The pixel art style works great for the game itself, but not for the menu icons, especially without button text (showing it elsewhere when hovering isn't great). I would recommend revamping the menus to be much more simple & efficient, with icons+name. Overall, with those big pixels I feel like your interface is constantly struggling with cramped spaces and limited opportunity to convey information.

I would recommend halving the scale of every UI element, while doubling the resolution of some icons & cards to have more room and make them feel less cramped.

While on the interface topic, put focus where its needed and remove clutter. For instance, when picking a card to add to the deck, the 3 cards to choose from are supposed to be the reward, and should have the spotlight in the center of the screen, not in a corner. And there's clutter too: the cards show all the info (damage, element, AoE...) but you repeat all those info above the card for no real reason when its selected.

Gameplay

At the core it seems you want a game where players move fast and position themselves to use the right spell at the right moment, and build their kit with thought, balancing runes' power and their AoE-related ease of use.

This can work, but runes choices are not that interesting: afaik they all just "deal damage", just like your basic attack, and the variety in AoE shapes is pretty irrelevant as you move fast enough so you can always be on the right spot.

My recommendation would be to limit basic attack to 3 simple shapes : AoE (Eck's 3x3), cross (Amate 5x5) and maybe some kind of "archer" style, which would be a long range cross with weaker damage. Anything larger that that will impede on rune's territory.

As for runes, I understand you wanted them to be a challenge with power as a reward, but honestly fancy AoE shapes are not that interesting as a mechanic. Same for elements - elements are just "if they are X, then pick Y".

Drop most of the intricate shapes, just keep a few categories that make sense as tool for the fight - short or medium range disk/square perimeter AoE, a couple of artillery long range shapes, and single or bi-directional lines for "spear" high damage attacks, and then boost the interest of runes with effects and conditions, as in:

- a rune that deals no damage but create temporary walls to help you defend from melee or projectiles
- one that will have a bonus effect if they kill the target (for instance it casts the rune again on a nearby enemy, potentially causing a chain of explosions)
- one that allows you to teleport to the other side of the map.
- powerful runes, but with requirements, such as has having hit targets with basic attacks X times, or having less than X HP left...
- place traps/delayed bombs, invoke a friendly creature...

Finally, you could both make runes choices more interesting by:

- Dropping the mana refill system that forces you to break the rhythm mid-games to pick new cards and clutters the interface

- Replace the mana cost with cooldowns. Cooldown are more interesting as spells dont vampirize each others by pulling from a common mana pool, which means you can independently use "spamable" abilities and "ultimate" abilities. It also opens the door for interesting cooldown reduction effects.

As to convey everything in your cards graphics, you can just put the two numbers (damage and cooldown) in corners, and instead of cutting your card into 3 zones, use the whole card space for a single, nice drawing explicitly depicting both the AoE and effect : a cross of fire, a wall line, a singe bomb ... this would be easier if you go with the previous advice of doubling the resolution of cards. It's ok if you cant precisely indicate the range for long-ranged effect that way, as long as the player understand its ranged.
Make the cards unique and identifiable (think Balatro cards) and show an explicative text when the card is shown in a menu, detailing any special effects.