Getting credit for game completion within a party

I was recently playing Castle Crashers with my two roommates. They hadn't yet finished the final boss, but my character had. We went through the last few levels that they hadn't completed, making sure not to skip anything. After completing the game, they didn't get the achievement and we couldn't enter insane mode. Is this because of some sort of level parity, or are you simply not allowed to beat the final boss if anyone in your party already has?

They created a game without me and were allowed to go directly to the final boss and after they beat it without me, everything unlocked as it should.

Solution 1:

Everything I originally wrote was true. However, there have been patches since and the rules have changed.

Based on recent testing:

• If I beat every level in order (regardless of party composition) I will get my Skull
• The above is true not just for every level but for certain "Check Point" levels:
  • Barbarian Boss
  • Compass
  • Industrial Boss
  • Dragon Boss
  • Ninja's (must leave from dock)
  • Vollyball Boss
  • Corn Boss (or Medusa, I didn't test which)
  • Level Before Final Boss
• Levels played out of order will not necessarily unlock that checkpoint
• Everything I just said is null and void if you have someone over level 99 in your party

Ok, so it took some testing to be sure but here is what I found:

• If I join a part with someone who has finished and I have not, I do not get credit for finishing (regardless of the levels I play).
• In the above scenario I do unlock various levels that we play together (and I did not have unlocked)
• To unlock insane mode I had to play through, by myself, the last level (with the boss on the floating crystal).
  • I didn't test if I could to this with a player who had not beaten the game yet.

I hope this helps