On this page of the game guide to Borderlands 2 you will find the walkthrough description of the main mission titled Best Minion Ever. This is one of the tasks that you will complete while progressing through the Southern Shelf area.
Southern Shelf - southern part
Southern Shelf - northern part
2) Gate which you need to destroy with the cannon.
3) Trapped Claptrap
Approach the Claptrap  and press the use button (by default E). Follow the Claptrap towards the lever in the western part of the camp which you can use to open the nearby gate (head further west afterwards). Follow the Claptrap to the west of the map towards the first small camp. There you will come across Bandits which you have to eliminate.
Now approach the bridge seen in the screenshot and wait for the Claptrap to lower it. With the bridge down, prepare yourself for a fight with Bandits.
Head across the bridge, jump below and prepare yourself to meet with Boom and Bewm. The first one will be operating the Big Bertha and the second flying around the area on a jetpack. You need to stay behind the cover seen in the middle of the area (as seen in the screenshot) and get rid of Bewm first.
After you get rid of him, start shooting at the cannon. Bandits will also join the fight. Wait for a gap between the shots and shoot at it yourself. Afterwards quickly hide behind a cover, for example the one seen in the above screenshot, found to the right of the last one.
Once you deal enough damage, Boom will leave the cannon. Finish him off while moving between covers. You can also run to a safe spot and wait for your shield to regenerate if needed. After the fight, pick up the Grenade Mod from Boom's corpse.
Now approach the gate in the northern part of the camp where the Claptrap will be standing. He will suggest using the cannon to destroy the gate . After getting rid of the gate, stay inside the cannon and attack the Bandits coming from behind it.
Once you don't have what to shoot at, go through the game and head north-west. That way you will reach a locked gate seen in the screenshot. It will rise after you approach it and Bandits will attack you.