To do a Shield Bash, just hold RT and then release it after a moment. You will thrust out your shield and knock back enemies, or even reflect bullets if you time it right.
The description is misleading, it should actually say "Shield Bash 1000 enemies". If you hit 3 enemies at once, it counts as 3 enemies instead of just 1 Shield Bash. Therefore you just need to get a group of melee enemies to attack you and keep Shield Bashing them away.
This should take no more than 20-30 minutes if you do it all at once, but I'd recommend spacing it out so you don't get bored. Note that you must hit an enemy, you can't just sit in a corner and Shield Bash nothing.
You can check your progress under Shield Bashes in the "Stats Book", it's the yellow book in your Inventory.