>>48171835Did both Colosseum and XD a little while back and I have some notes to share
Colosseum is probably the rougher of the two since you have fewer options, the level scaling can be really rough at points and the early game is hard, but it's certainly doable. The worst part is between the beginning and when you have access to Mt. Battle since not having access to a reliable place to grind is really hard, the Duel Square becomes obsolete as a grinding spot almost immediately. I would straight up not recommend you use your Shadow Pokemon at all before you can purify them, opting to use Espeon and Umbreon exclusively since they're the only ones who can actually level. I tried using a full team on my first try and quickly ended up losing half my available mons before I even could purify them, my second try being much more successful where I mostly relied on Espeon and Umbreon until I could purify the others. Another thing to point out is that although there are some jumps here and there in difficulty, by far the worst one is the first Venus fight, going from high 30s to two battle in the low 40s to her battle in the mid 40s, this is where I wiped my first run because my Pokemon just weren't high enough level at all. Thankfully you have access to Mt Battle by this point, so it's just a matter of grinding up a little bit and not getting cocky by going too high. Oh, also, DON'T TAKE ON PYRITE COLOSSEUM AFTER THE FIRST MANDATORY CUP, THEY IMMEDIATELY SCALE TO LEVEL 50
XD is in some ways easier than Colosseum but in others MUCH rougher. You have access to purification and Mt Battle pretty much right away and the level scaling is better, but the game is much longer and the moves and strategies the opponent uses are much better. The first half of the game is pretty smooth (except for two points, I'll mention those), but Citadark Island is just an absolute nightmare, I would constantly have to stop, go all the way out and grind up new Pokemon because I lost something