No problem.
Get a metal dustbin and a humane rat trap.
Bait the trap with peanut butter and wait.
When you’ve caught a rat, put it in the dustbin and reset the trap. Provide water but no food in the dustbin.
Every time you catch a rat, deposit it alive in the dustbin. As the rats in the bin become hungry, a cannibalism instinct kicks in. It’ll be like the Royal Rumble in there.
After a few weeks when you’re left with only one rat, carefully release it back into the garden.
He will be the undefeated champion, he’ll be bigger, stronger and more vicious than all the other rats and has been existing on a diet of pure rat protein. He will also have a penchant for rat flesh, so will cannibalise any new rats that may stray into his territory.
Just keep him away from small children and family pets.