There are three basic attacks. They are triggered by pressing down, quickly followed by another arrow.
Down+left and down+right are basic attacks, which make arrows appear on the opposite side of the arrow you pressed (i.e. down+left will push somebody to the right of you rightwards, and down+right will do the same for the left).
Down+down is used for blocking. Blocking is useful for interrupting somebody's jump (to make them fall down), but can also be spammed to double-jump (technically, it means you jump on the block before it disappears).
Down+up spawns arrows on both sides of you, pointing inwards. It can be useful to get campers out of corners.
Basic attacks have a 1 second cooldown.
Special attacks work a bit differently. They can be purchased in the shop (the lower part of the hub) for coins. You can store three special attacks at a time: one triggered by pressing up+down, one for up+left or up+right, and one for up+up. You will have to memorize the type of attack, however most attacks work quite intuitively. You cannot keep special attacks between sessions, so use them before you leave (or before the bot is closing)!
There are a few ways to obtain coins. The most straightforward one is to grab coins that spawn on the field: blue coins are worth 1 coin, and golden coins are worth 5. You also get 25 coins for winning, and you can get up to 50 coins from chests!
Chests and other containers appear during the battle. You can open them by pressing a certain arrow twice while standing directly next to or on top of it (or under it, if you manage to get there somehow), depending on where you are (i.e. double down when you're on top of it, double left if you're on the right, etc). There are different types of containers, and some may give you items, while others might give effects!
Every now and then, a Smash Ball appears (the viking shield decoration), floating around the screen. You hit it just like a container, although it will be much harder, because it isn't solid and floats around the map freely! If you manage to hit it, don't get happy yet—you need to hit it once more to destroy it! If you manage to get it then, press down+down (it will replace your blocking move) to use your Final Smash. Keep in mind that every smiley has a different Final Smash!*
The power of your basic attacks (and a few special ones) is determined by your tier. Your tier is determined by your amount of wins. The higher your tier, the better and more effective your basic attacks will be! Of course, it won't be impossible to defeat people in higher tiers, because of the presence of large maps and Final Smashes.
If the round hasn't ended after 2.5 minutes, the screen will fill with lava. The lava will rise by 1 block around every 5 seconds.