I always play on Proud for
KHFM and
Re:CoM, and Critical for
KHIIFM,
BbSFM and
0.2.
With
DDD, I've played Critical a few times, and it gets easier each time. But the trauma of Ansem 2 on the first Critical playthrough keeps me from wanting to do it.
I've also done LV1 Critical on
KHIIFM twice and LV50 Critical on
0.2. They're fun and challenging.
BbSFM LV1 Crit Aqua was pretty easy, but I know she's the easiest because of Spellweaver.
I don't really feel like playing Ven or Terra at LV1, nor
DDD. The way those games work, all LV1 does is underpower your stats which only makes fights longer, not harder.
KHFM LV1 Proud is literal hell and I don't ever want to do it again.
Oh, you mean am I okay with cheesing? Absolutely, as long as it will get me that sweet lookin' keyblade.
And I've never seen that before! I'll check it out! I think you'd need to threaten me before I ever gave any of the secret bosses a try as Terra, however.
Specifically, use Tornado underneath Vanitas when he does the X slash in the air. Do that I think three times and he's dead.
You can also use Mines to launch him if you're quick enough and he doesn't teleport.
For Terra, I had a very easy time using the Demolition Finish command.
It might be tough to use basic attacks to fill the gauge, but once you have it, you can deal massive damage to Vanitas while being completely invincible. VR will constantly try to attack Terra, making for easy potshots. Only takes about two Demolitions on Critical.