Boosted leveling builds teams of 2 x 25 pets + 1 level pet. With only one it can not work. But even with 2 you are looking at a tough time. Say you are leveling a level 5 pet against level 25 enemies. You want the bot to beat 3 level 25 enemies with your 2 level 25 pets. You will be constantly waiting for your pets to be healed.
For new pet battlers I always recommend the same: Level one rare pet of each family (but 2 beasts and 2 flying pets) to 25 with relative leveling. It takes a while, but once you have them at 25 you have a good variety of pets for relative leveling. Then your level speed increases significantly without a lot of downtime.
According to the docs, the system would not be battling level 25 pets during boosting, but would drop down to an appropriate level. Say level 11 battles to boost level 6 pets with a single level 25. But if you say I need 2 level 25's for boost leveling to work, I will head in that direction.