Crimson Alliance

For starters, the text for this achievement is incorrect. Not only do you have to get a gold medal on each campaign level, but you also need to do the same on each of the challenge maps as well. And it must all be done on the same character (I made that mistake myself and had to redo a few levels). I highly recommend doing this achievement on co-op but it is possible to do it solo, just much harder.

For the campaign level you will need to play each of them on the Ridonkulous! difficulty and get through the level as quickly as possible. Time will be your biggest score increase in these levels, your combat score isn't very important. On the Ridonkulous! difficulty you should get gold medals with ease and more than likely you'll end up with platinum medals.

For the 2 boss fights (levels 5 and 13) and the challenges maps, you'll want to turn the difficulty down to Immortal. In these levels your main focus needs to be combat. You will need to rack up as many points as possible using the kill multiplier before finishing the level. Most of these require you to complete them without getting hit a single time.

The last boss can be easier to get a high score if you stay in the first phase of the boss when the zombies continuously respawn. Unfortunately, this is the only level that this happens in.

-- Solution solely for Storm of Blood and Asturi, which are the hardest levels, single player methods!!! --

Storm of Blood:

Difficulty: Normal
Class: Assassin
Equipment: Winterblade + Zeeker + Gadget Master

Method:
Now, the level is all about speed combined with a high multiplier throughout (x5). The toughest enemies are the shield enemies, because they steal a lot of your time. That's why you have the Gadget armor. It randomly gives you turrets for killing enemies. Use Y (freeze) a lot and kill enemies only with X, because it's your strongest attack. Hopefully you get a lot of turrets that just waste away the enemies and luckily kill shield enemies from behind! Also, the turrets build up your multiplier fast! I didn't even use any special items. Use your power attack (LB) on the second-last wave.

Got over 6000 points (5500 required for gold) this way after only a few tries! This method is all about lucking out with a constant spawning turrets that waste away the enemies and spawn new turrets while your job is to not get hit too much, freeze enemies and kill what you can yourself.


Asturi:

Difficulty: Normal
Class: Assassin
Equipment: Winterblade + Volt + Life Stealer

Method:
You need 8.000 points for gold medal. All you need to do is stay back and shoot the infinitely spawning skeletons and zombies in the first boss wave with your B attack and move around if the boss shoots his magic balls, which are easily avoidable. You will build up a high multiplier and get to 8000 points in about 10 minutes. If you got the points, just play it safe and kill the required enemies. Since you have the Life Stealer armor, you constantly get life back for killing enemies in case you get hit. Since it's normal difficulty all you need to focus at are the points though. This methid is very easy just takes time - a lot less time than constantly retrying harder methods though! You also can get the 8x multiplier achievement this way!

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This is my first solution so bear with me here, I do give credit to Fliptriq on his part about the ridonkulous difficulty through the game.

Now here's what you do (for the last boss)

Set the difficulty to normal, and when the last boss fight starts don't try to take down the guardian mages that protect her, instead keep attacking the minions that spawn trying your best not to get hurt, (tip: use the mages freeze to help you not get hit much). Once you have 8000 points, just take out her guardians and finish her. theres an easy gold or even platinum.

Now for all the other levels it is best to have a partner but you can do it solo. Play through every level on the Ridonkulous difficulty and try to build a multiplayer without getting damaged, my friend and I did every level with a platinum on the first try, this achievement isn't as bad as people say.

Note: (the last challenge go on the immortal difficulty and try your best not to get hit, I found the mage to be the most useful as you can attack while using your temporary shield.)

Please dont vote negative unless you give reason, so I can improve =] hope this helped