I wouldn't say it's "difficult" at all. I *would* say it'll tax your patience to the nth degree. That's because the Guitar Star skill challenge relies heavily on luck.
For those not familiar with this challenge, the skill book says you must perform at 10 venues or parties. What the challenge actually requires is that you accept and complete 10 Skill Opportunities that take place at parties or venues. In other words, you must wait for those phone calls, hope they are Guitar Star Opps (that's the luck factor) and if so, complete those Opps.
To make things a bit fuzzier, the game does not provide any tracking for your progress (2/10, 3/10 etc.). Unless you keep track manually, you won't know how well (or poorly) you're doing until you get the Congratulations notification.
You can stack the deck a* little* by keeping the Skill Opportunities slot open and by minimizing the number of other skills your Sim learns. Your best bet would be to have no other skills and to reject every Skill Opportunity that cannot be completed immediately so as to keep the slot open. If the only skill you have is guitar, 100% of the skill opps you'll receive will be Guitar opps. That's not to say all of them will be Venue opps but the chances are better. The moment you start other skills, your chances of getting Guitar Skill Opps (and thus Venue opps) decreases because you've now diluted the pool with other types of skill opps.
As I said, it's not particularly difficult. It requires no special gameplay skill, but in trying to complete that challenge you may need to pass up other things. I suppose it's a matter of what is more important to you.
