Abaddons mouth.

What skills do you suggest I use as a ranger/mesmer? is there anyway I can help attack the people on the wall near the start?

Well, interrupts really help vs. the casters in that mission, especially the boss Mursaat monk. Depending on what your build is, you could use some of the mesmer skills that ignore LoS to help out with the wall at the start, but they'll be just that...help. For instance, if you're running degen, you could drop phantasms, or there's plenty from the domination line, really. If you're playing with henchmen, grab Cynn and be patient. If you're playing in a group, grab a nuker, or several other characters that can ignore LoS, such as blood necros or domination mesmers.

Los..?

Line of sight...arrows will be blocked by the walls but spells can still hit.

As a ranger you are definately going to want some interrupt skills. It would also be nice if you were a necro secondary to bring spinal shivers along with an ice stringed bow.

If you have an elementalist in your group or a warrior using an elemntal weapon winter is an excellent choice for all the fire island missions, even without spinal shivers in the equation.

Having just done the mission/bonus/all caps with my mesmer, Cry of Frustration is a dandy skill to bring along to slow down the Jade Bows/Armors

Other interrupts are nice, but I was more limited to interrupting the spell casters.

Backfire on a Mursaat Necro kills quickly. Guilt, Power Leak, Power Spike, Power Block the rest of them. All of these are from the domination line I believe, so you won't have to divide points up too bad to be effective.