If you want to relate the four-person party EL to a one-person party, you can derive that from the guideline +2 EL for doubling the number of foes.
To have a challenge equivalent to an 18 EL, you would need one CR 14 opponent (equals EL 14 --- four CR 14 opponents would raise the EL to 18 (two doublings in number result in a +4 EL) --- EL 18 is a proper challenge to a four-person party of 18th level, so EL 14 is about the same for a one-person party), or some mix of opponents, that result in an EL of 14.
You can probably go as high as 16 or 17, if you want it more challenging. At EL 18, chances are roughly 50/50 for the PC to survive.
But don't forget, that these are mere guidelines. A single PC might lack the necessary abilities to deal with such an opponent, while another will have it incredibly easy. The CR/EL system also assumes a fair mix of abilities.
Some judgement on your behalf is always necessary.
Bye
Thanee