Overview:
-Estimated achievement difficulty: 4/10
-Offline: 11(190)
-Online: 1 (10)
-Approximate amount of time to 200: 2-3 hours
-Minimum number of playthroughs needed: 3
-Number of missable achievements: None!
-Glitched achievements: None!
-Do cheat codes disable achievements?: No Cheats

The perfect game for anyone old enough to remember it. The achievements in this game are pretty simple except for Perfect Win but can be done in co-op which simplifies things a lot.

Step 1: Three plays.
It's recommended you complete the game at least once to learn what the bosses do, how to take/avoid dragons etc. First of all, you'll want to start a new game with each character. With each character, fill up your potion slots to the maximum and use your magic when you've done so. Now you can either carry on or die, either way you'll get the three spell achievements and on your third go you'll get the Omni Character achievement.

You should also pick up:
Cockatrice - 5
Potion - 15

And if you chose to finish the game with a character, you'll get the Win achievement for 35.

Step 2: Co-op.
Now use a second controller to go into local co-op and on the first level die. Or if you wish of course, complete the game in co-op. For the second co-op achievement go here and find someone willing to play it with you. Again you can die right away or complete it.

Step 3: Perfect Win
The only achievements left to get should be:
Turtle Village - 10
Survivor - 20
Perfect Win - 50

These should all be earned in your final playthrough in which you should put the health/lives up to 5 each. I suggest going here again and asking somebody to help you get Perfect Win, and you'll get Turtle Village and Survivor on the way.
Use your buddy to take hits for you and generally protect you, this achievement will come easily if the person protecting you is 'okay' at the game. Meaning, they won't hit you instead of the enemies or head butt you off a cliff.

Conclusion:
I said that the minimum plays is only three but listed six. This is because you can combine each play with each character for different achievements.
For example:
Play one - Barbarian, online.
Play two - Amazonian, local co-op.
Play three - Dwarf, Perfect Win.
That being said, I recommend doing the six plays to get good at the game to prepare for the Perfect Win, you don't want to get on the final boss only to die lots and use a continue. The plays don't take too long and it's a fun game so it won't feel like a chore.

[x360a would like to thank chrisnottm for this Road Map]