The Boss Rush arena is available as soon as you enter the Southerness area. It's through some flammable bushes to the right, over some cliffs and in a tower with some hard to climb stairs, (stick to the bottom, as shown).

While you may want to wait until the end game to tackle this, it's entirely doable early in the game. The reward is lots of coins, and as long as you're patient and play defensive, it's not too difficult. The reward will be useless at the end of the game, as money is only used for upgrading your attacks. (You only need 2300 coins to purchase all the things from the shop.)

You only have the one health bar to beat 6 bosses. While they will drop hearts, it won't heal much, still, if you play it safe you shouldn't have too much trouble.

A Big Haggis.

It charges at you, just be careful minding your range, as it can hurt you even if it's charging and you're behind it.
It's probably the most dangerous boss, as it attacks quite unpredictably, is hard to dodge, and does a lot of damage.

A Big Fire Sprite.

Charges leaving a flame trail and explodes.
Just run away, then wait for it to finish exploding before giving it a few hits, before backing off.
Should be easy enough if you don't stand in front of it.

A Big Spitting Haggis.

It will stop moving before it attacks, that's your que to run in a big circle to avoid it's attack.
Easily the easiest boss, as you can attack like mad, then as soon as you see it's walking animation stop, you back off and it's attack is easily dodged.

It's Gregor and Calvin McKinley, again.

Will walk up and try and hit you, but it's fairly easy to dodge and get a hit or two in.
They'll then do an AOE attack you can dash over. They'll launch 2-4 of these before their final attack type:
Running around in a square leaving a trail of fire and closing in on the centre.
Usually best to avoid getting trapped and just throw rocks from a safe distance, or leave a tornado in their path.

If you play it fairly safe, should be easy.

Another, Big Fire Sprite.

Charges in a zig zag fashion, leaving fire trail before launching small bouncing fireballs 360 degrees.
Be patient and wait for an opening. It stuns easily, so look out for the dizzy, spinning stars, then smash it.
If it recovers from the stun, back off and wait for another opening.

Yes, it's Evalyn, again.

She'll charge at you before spinning in a circle and firing off lilac missile.
It's best to keep distance so you can easily dodge these.

After this attack she will fire a semi-circle of harmful slime on the ground.
You can take this opportunity to get behind her and hit her up, or down, away from her slime.
She'll then launch lots of lighting at you. Again, best to play it safe, but if you bash her right, you can avoid the lighting by moving and attacking. If you start getting hit, best to back off completely.

Evalyn will then pounce at you.
Keep moving so she misses and get a hit or three in, but watch your distance as she will start squirming, and stomping.

Enjoy a big chest of coins and hearts.