You can meet with a Hero Corps Agent at several places around Paragon City and tell them that you are tougher than the default "Heroic" setting. This word will spread, and as a result, your missions will be more difficult.

• up 1 notch: larger groups, same level mobs
• up 2 notches: same size groups, +1 mob level: "Rugged"
• up 3 notches: larger groups, +1 mob level: "Unstoppable"?
• up 4 notches: same size groups, +2 to mob level: "Invincible" ?

You can also, if you find the missions too difficult, move the 'slider' (which isn't in any way shape or form an actual slider) back down to "Heroic," a.k.a., "not as tough as I thought I was."