Awesome game. Very challenging, totally unique and fresh.
Here's some tips. Learn how the game works. There is a ton to take in, but the game is so addicting it only takes one session to learn all the hidden rules. Think 32 bit arcade Dark Souls Co-op.
A tip on movement. If you need to gain some distance from an enemy to fire ranged shots. Click to a different room, (away from mobs) than reclick the room that you and the mobs are in and you'll turn back around and start firing.
Criticism: Game is likely at it's funnest when playing with competent friends. However, there are a few functionalities that put a direct negativity to multiplayer.
1: Not being able to save the game while in multiplayer is dissapointing. I agree it shouldn't be so simple as "oh lets save a 3 player progressed game and I'll just finish it later by myself." But we were at it for 3 hours and only got to the 7th level of the 12 levels on the first dungeon. Now the idea is kind of cool to do it all in one sitting, which if you can do, that'd probably be best. But some gamers are workers too, and might not be able to commit 6 hours for on session game. What I'd like to see is a "group save" function, where the save file is only accessible if all involved players are present.
2: Keeping criticism 1 in mind with how long the session can last and how you can't save, you also can't re-join any game you somehow DC from. 3 and half hours of progress then "poof." Gone. You lost the hero you almost earned, and your mates will have to try and carry on without you. This happened to me and it was extremely dissapointing (because this game is extremely good.) After surviving hordes and hordes of some of the most difficult mobs, no one wants to die indefinitely to server problems.
3: It's far easier to earn new heroes playing solo, since in a party of 3, there can be only one more hero added to the party and can only be controlled by a single player and only that single player will earn that hero. Which from a different perspective may seem like "only the ones who earn the hero get the hero" but to me it's just a reason to not play multiplayer, which this game shouldn't be doing.
In summary, embrace the multiplayer. Make it a bit friendlier. Other than that, awesome game. 4/5 stars.
One more thing. The 32 bit art in this game is gorgeous.