New builder

Well for 1 * creep wave - level 8 is a below average difficulty levell especially after level 7 (because level 7 is a tough wave you can normally full worker/lumber upgrade on lvl . Level 14 is an above difficulty level for most race unless you have 2 spawns (for beast, I build 2 spawns before 14 (1 one 7 and 1 on 13) I like to use this build, so you can full worker 14 and 15 even though level 13 is tight (and you may leak a few) because spawns take heavy dmg from magic attack and dont splash air, but with their good stats they still manage to do a decent job).

for 3 * waves - level 8 is definitely an above avg difficulty level (ranged levels are much harder in 3* waves because nearly all the units can attack at once and alot of them target your units 1 by 1) level 13 is also above avg diffuclty because air has almost no collison size so they attack at same time.
Because you generally have to overbuild a bit for level 13 level 14 is generally easier.