But what game are you talking about? =/ I'll assume Colosseum, as you mention Colosseum mode...
Anyways, their Pokemon will from memory match the level of your highest Pokemon - with a minimum at level 50. E.g. you bring a team in which the strongest Pokemon is level 67 - they will all be at level 67. Bring a team in which the strongest is level 30, and they will all be at level 50.
It's nothing hard, really, exepct maybe towards the end. I had very little trouble though, with an un-EV'd team of level 50's including a Quagsire. -_- It's mostly the fact you have to do 100 battles, really. Good luck, and if you aim for the Ho-oh you get for beating Mt Battle in Colosseum mode, you HAVE to purify all of your Shadow Pokemon first. (Otherwise you won't get it).
And I'll just say that for the in-game mode...
For Colosseum, they are all at level 60. A link to a guide which lists all the trainers and their Pokemon as well:
https://au.faqs.ign.com/articles/765/765747p1.html
And for XD, it's at level 70 by the time you get to battle 100 (note Lati@s are two of the Pokemon in the final fight -_-). A guide with all Pokemon in XD's Mt Battle here:
https://www.gamefaqs.com/console/gamecube/file/925945/39940
Hope that helps. =)