Gross Domestic Product of the United States stands at a staggering 23.32 Trillion. China is not that behind. But still there are some measures that NON allies may take that can reduce the size of the US economy.

Fiscal Policy:
If somehow their government spending increases. In this case the Ukraine war. And the spending on the Pandemic and other outbreaks.
If their taxes are decresed. Or due to a lack of trust in the government the publics starts evading taxes.
Monetary Policy:
Decresed Interest rates to increase borrowing and spending. Research from financial services company Northwestern Mutual found that excluding mortgages, the average personal debt per individual currently sits at $21,800, significantly lower than the $29,800 recorded in 2019.
Regulatory Reform:
Difficult regulation will hinder business growth and investment.
Trade Policy:
Alienate the USA to decrease their exports and create economic barriers.
Industrial Policy:
Discourage diversification. No new industry is developed. Create ample hinderances. Increase their dependance on a single sector or commodity.
Labor Market:
A Decreased flexibility in their labor market that can hamper worker protections.
They start discouraging workforce development. And stop investing in education and training programs that will significantly decrease the employability of their work force.
Privatization:
Privatization stops. The government owns everything. Increasing the burden on the government budget.
Social Safety Nets:
China needs to discourage welfare by the world in the USA resulting in a Squeezed welfare programs.
International Cooperation:
Alienate the USA completely! Unemployment and economic instability in the USA are your friends.
China and Russia need to create these teams ASAP. You can hire or bribe the saleable democrats and republicans. And use them to your advantage.
It is a humoungous task to destroy a country with a big GDP. It is going to take the smartest Russians, Chinese and Pakistani brains to do so.
Step by Step and brick by brick you can create a divide and rule this world. It is not going to be easy, but it is also not impossible.
Play with them from the inside!
Period.