With Trample it works well. Even without double strike.
You attack and he blocks, now he/you removes the blockers from the combat. Now you have an blocked creature, but with trample you only have to assign minimum deadly damage to the blockers (in this case 0) and then you can assign the rest to the player.
With double strike you can to this 2 times. The first time in the first strike combat damage step and the second time in the normal combat damage step. So it will be 2*6 damage=12.
The Bold thing is not correct. Blockers are removed when damage is dealt by Pit Spawn, that means the first damage that is dealt has to be assigned to the blocking creature(s).
After evasion, Trample is the next best thing to kill by Phage.
If the power of the attacking creature > toughness of the blocking creature(s) then you don´t need tricks with First Strike/Double Strike.
If the blocking creatures have toughness > power of your attacking creature, you could make up a trick with a trampling /double striking Volrath´s Shapeshifter, with the idea to eliminate blockers in the first strike combat damage step and kill you opponent in the normal combat damage step.
But why make life so difficult? Dreadnought tramples over (almost) anything.