You can’t get Garchomp from any NPC’s in B2. In fact, you can’t even find Garchomp, Gabite or Gible in the Wild. The easiest( and only) way to get Garchomp in B2 is by clearing the hardest trial of the game.(black tower, trust me it’s really hard to do without good preparation or grinding. )
After (if) you do manage to best Benga in a Pokémon battle on the 10th stage of black tower, you can get a level 1 shiny Gible. While it is shiny, you will need to grind really hard to get it to be Garchomp. By then it will look like a regular Garchomp and you will have wasted 50 hours of your life.
However in Gen 4 and 6, Gibles are really easy to obtain. And you can obtain them before the E4.
“Don’t take this entire thing as a diss on black towe, all my 6 Shiny Gibles come from Black tower...”