There's a couple of options, one is that there's a proficiency without level option that just drops level to 0 on everything, but ive heard that isn't very good (I wonder how it would play with half level.) You can also use the tables in the GMG to translate everything manually (I actually have a plugin in the VTT I use, Foundry, that does this for me) to whatever level you please by finding the number it has at whatever level it is on the table, and then checking the corresponding category for its new level. Finally, you could work around it by accepting it as a reality of the game world that there's some foes you just can't touch until you're ready.