Marvel Ultimate Alliance

You should have a good bit of coins to help level up some of your weaker characters' costumes to help with the sim missions if you've done "The Destroyer" and got 100 Arcade wins. You'll want to focus on defense and attack before anything else. If you have a suit with XP bonus or something like that, don't bother upgrading that stat. You can also play around with the equipped piece of gear, but that won't make a huge difference.

To get a gold on a character's sim missions, you will need to hit a target point total. Most of the character-specific missions have a set number of enemies to kill, and the deciding factor will be how long it takes you to complete the mission. Make sure you stay in each room until enemies stop spawning, but move as quick as you can otherwise. If you can kill every enemy and complete the mission in about 8 minutes or less, you'll be in good shape.

Getting gold is much easier on these three levels due to infinitely spawning enemies: Murderworld (bumper cars), Asgard (anywhere before the boss), and Omega Base (console room). Simply spend a few minutes here and get all the way to your point goal (or perhaps 2-3k under) and then finish the level. Even if you take a long time, you'll get a few thousand for your time bonus to help out if you stopped killing enemies prior to the goal.

The only really annoying simulation was Captain America's for me. For the Winter Soldier fight, make sure you reset the countdown to get more time. You can't kill him in 40 seconds, so just attack for about 20 and then reset the timer (you'll need to run around and hit three of the consoles), then continue attacking him. The rest of the challenges, especially any of the "Vs" challenges, were pretty straightforward. Always upgrade your suits when possible and pump all your skill points into one major skill that will do the most damage and you'll be golden (pun intended).