Originally Posted by Natanji
Actually Tavvy, the difficulty of the boss depends only on the shotdown percentage you get in a level. Of course if you use inifinite overdrives, that will always be close to 100%, thus getting you the harder "Tera" version of the boss
Teehee its funny you commented me this because I realized that today while I was playing score attack to get the last achievement.
On my first run on Mission 1 i got around 99.44% (I missed 1) and two rings appeared and made the boss hard but then I did it again and got around 97.50% and only got 1 ring round the boss and was easier!
So I think to get the harder bosses you have to get around 99% + or only miss around 1 or 2 enemies
But anyway thank you for correcting me!!