How do I become better at competitive battling?

Lets see well one thing I use is the one-hit KO moves Guillotine,Sheer Cold,Fissure and Horn Drill!

Or for example if using something like Gyarados or Milotic ice type moves would be a good thing to teach them and high speed and attack stats also work this is how my Kyogre,Groudon and Rayquaza kick butt without trouble and moves like Extreme Speed come in handy or Skull Bash and two turn attacks such as Dive,Dig and Fly are also great to use.

Here is also a plan:
Use something risky such as Outrage or Petal Dance.

Or Brave Bird,Take Down etc.

Oh and stuff that effects the status of a Pokemon moves like Thunder Wave and Sleep Powder can be quite an advantage.

For Garchomp though Ice and Dragon moves are crucial especially the Ice type moves seeings its Dragon/Ground dual type

I have a strategy that I use...I get a flying type and a water type make sure the flying type moves first! Use fly for the flying type and then use surf for the water type (this can work well with a ground type instead of a flying type also) use Surf with the water type (or earthquake if ground type) the flying type will avoid the water (or ground ) move and after that the flying type will come out unharmed

You run instead