I am heavily abusing naval invasion mechanics and how they absolutely brainfuck the AI. I still almost lost this last war (liberating New Africa and taking Pennsylvania) because Great Britain's idiot fleets were set to "defend naval invasions" and wouldn't help me
The secret to killing the USA early is to zerg them with rapid advance cavalry stacks, they only start with 10 regiments so you can crush them into dust easily. Always take Pennsylvania and New York, they're dead without them
I've been able to take them out early with all sorts of minor powers with this strat. One time I annexed or puppeted all of the USA as China by 1855