Check it out. An Achievement Hunter map!
http://maps.achievementhunter.com/guides/overview/grandtheft...But here's some tips. For the under the bridge ones you will want a Buzzard so buy one from the Warstock website or take it from the Army Base. You can get into the army base without drawing any attention to yourself when you're doing the 2nd Cletus mission when he teaches you how to hunt animals instead of satellite dishes. But if you've passed the mission up without doing it (like me) then your best bet is to fly into the army base. There are 2 places I've seen where they spawn, in front of the Eastern and central hangars. If you try this then abuse that quick save function. Save as you fly in, save as soon as you get in the chopper and save multiple times as you leave. From my experience the fastest route is to fly the chopper south to the runway, then west away from the SAMs and Rhinos and fly over the fence before the bridge, staying as low as you can to not get shot down. Every time you load up the enemies have to respawn and you have about 100 Meters around you clear.
For the knife flight, buy a mallard or wait for one at Trevor's airfield in Sandy shores. They land occasionally, alternating between that and the crop duster. Or buy it from Elias and store it for future use.