How do I shorten Project Duration and Level resources

OK so I've downloaded your file and checked it out.
In a nutshell, in order to reduce duration, I think you will agree, you need to increase resource.

Now the first thing you need to do is add the "Work" column.
Right-click on the "Start" column and select "Insert Column" then select "Work" from the drop-down list.
This will give you the actual work hours spent on the tasks (note these are very different from the duration hours)

Now that you have done that let's look at the Site Preparation task to start with.
You have this task taking 182hrs of actual work (total combined resource hours) but lasting for 7 weeks (280hrs)
Looking at the resources assigned to the task you have the following:
PM: 10% utilisation (i.e. 28 hours of work to complete over 7 weeks)
Facility Specialist 20% utilisation (i.e. 56 hours of work to complete over 7 weeks)
General Contractor 35% utilisation (i.e. 98 hours of work to complete over 7 weeks)
Total Resource Hours: 182
(Note that the Site Preparation resource is irrelevant for determining work and duration as it is a fixed cost)

So we agree that in order to finish the task quicker you need to utilise the resources more.
So if we double the utilisation of the resources then we should halve the duration so...
Open up the task again and double the utilisation for each resource so that:
PM: 20% utilisation
Facility Specialist 40% utilisation
General Contractor 70% utilisation

Now you will see an exclamation mark appear at the left hand side of the task wanting to know how you wanted to effect doubling the utilisation of the resources.
You can do this in 2 ways:
1) by making them work more often (i.e. Work hours remain the same but duration in which these work hours are completed reduces)
or
2) by making them do more work (i.e. Work hours to complete the task increase but duration remains the same)

Choose the first (and default) option to "Change the Duration..." and hey presto the duration is reduced to 3.5 weeks.

Bear in mind that resources work in parallel, so in theory it would be possible for all 3 resources to complete their allocated work in the time it takes the resource with the most work to complete their work.
i.e. the General Contractor resource has 98hrs of work to complete on this task so in the time it takes them to do that the PM could easily complete their 28hrs of work as too could the Facility Specialist complete their 56hrs of work.
So in theory the task could complete in 98hrs (2.45 wks)

Hope that makes sense?