my pet is level 3 and now should be level 4 but i cant level it up.
whenever i click one fuse, i lose another pet and got text "failed".
now my friend told me that i have to fuse it with upgraded pet, but i dont understand
should another pet be maxed with xp like 43200/43200 or should i level it up to level 2 and then fuse it with level 3 to make it level 4?

You have to create another lvl 3 pet and fuse with that, and it can get fail anyway.
I dont know if it works with lvl 2 pets, but if it works it must have very low succes rate.

leveling to 3 is fucking boring i cant do that again, take too much time and school will start tomorrow so i can only level it to 2

meh, i just buy lots of pets and when i get a skill i didn't have, bam. Keep it. But when I get a pet that has the same skill (or a useless skill) as one of my other pets, bam. Deleted

Yup i do the same way , just made a pet lvl 2 for the looks xD