I wouldn't say i am a MSC expert as I have not unleashed my potential online (still waiting for a Wi-Fi USB for my Wii). Anyways there are few tips I can offer based on the way the cups are designed and how you can achieve better status in Road to Strikers Cup and also online.
You know you are too weak to play Fire Cup when...:
1. You can't beat a Rookie leveled CPU in Domination Mode
2. You have no idea what team is best or how to form a good team
3. You have NOT completed Striker 101 (optional)
You know when you are ready to play Fire Cup when...:
1. You have succesfully beat CPU's in Domination Mode and can freely apply the Megastrikes and Skillshots.
2. You can move about, tackling and getting the ball with no problems
Tips: The Fire Cup is very easy! the oponent CPU's BARELY ever attack you, thus you are able to perform 2~3 easy Megastrikes per game, making it a 10 goal average per match. This goes for the elimination rounds and the semifinal and final qualifiers.
Bowser Jr. (BOSS)--> Map: The Lava Pit: Don't worry, he's match for you! Avoid walking on open lava pits even if the ball goes towards the crack. If you wait a while, the lava will rebound the ball giving you the time to grab it and head for some goals.
You know you are too weak for Crystal Cup when...:
1. You cannot beat professional CPU easily in Domination Mode
2. You cannot MATCH a superstar CPU in Domination Mode
3. If you were relieved about beating Fire Cup
You know when you are ready to play Crystal Cup when...:
1. You can beat superstar CPU in Domination Mode with eez.
2. You can match up to Legend/ Megastriker leveled CPU in Domination Mode
3. You can quickly form strategies based on how your oponenent plays
Crystal Cup is, a harder cup than Fire by a mile. It requires skill and a defined strategy. My first walk through the doors of this cup were quite humiliating. I lost several matches in the eliminations and had to restart the cup due to losing. It is important to get a strategy (like a lobbing technique) to get around your oponent. After much practice and being able to use the dekeing technique for various characters properly AND have developed a master plan strategy, I took on the Crystal Cup champion. And....lost the first 2 times.
Here's how you CAN beat Diddy (I have and was very happy)
Map: Crystal Canyon
Tip: This map is very annoying as you DO NOT know how you will end up and what strategy to use as some or one of your characters are burned at the begining of the game (same goes for Diddy's). If you do not have any fast characters remaining, your in some trouble. Make sure you have made a balanced team before doing this cup or you will lose. USE ITEMS WISELY and you will beat Diddy. Shells and bob-ombs are essential to beating Diddy in this 2 round competition. By bombing your oponents characters, you are safe for skillshots/ megastrikes. Diddy's special move is called "Red Card". He sends a teleportation ray that removes your character or any character that walks into the ray, out of play for a short moment. To avoid him doing this, don't takle, slidetackle him when he or his teammates do not have the ball. This will require some practice for those who love wriggling the wiimote on and on trying to tick off their oponent.
I really hope this guide has helped you guys through the difficult cups. I have not yet come across doing the Striker's Cup yet. But if I do, i will try to make another guide to help you guys.