Colress I thought was pretty easy in the battle at the frigate. In the post game rematch he has a 70+ team with Pokemon ranging from Metagross to Rotom wash, but for the final battle in the main story that you must do I don't recall any problematic areas. Half of his team is nfe, and the levels are pretty average. They were all but one steel types, leaving them susceptible to the same weaknesses, and in the case of the magnemite line 4× weakness.
Ghetsis wasn't hard to me either in BW2. Now the original Black and White Ghetsis is a different story. He ripped through my team with just Hydreigon. Even with Reshiram in my party I was only able to salvage the battle due to max revives. His signature pokemon is fast, and hit like a truck with a great special attack and some really unexpected coverage that hit at painfully high base power, fire blast, focus blast, surf etc.
BW2 however lowered the levels of his team, changed Hydreigon's entire moveset to remove any coverage, reduced the basepower of every move and didn't give him a single special attack, forcing it to rely on only physical moves which drop his attacking power by 20 pts. Hydreigon doesn't even get an accurate stab move. With crunch, dragon rush, rockslide and frustration it looks like he purpousely built this pokemon so that it would be walled by Lucario, which you can pick up by getting a riolu early game. Honestly, I think what they did to Ghetsis was cruel...