Ok if this is an exploit or trick BUT! I just wanna contribute.
1. Summon your demon Outside AV.
2. Enter AV
3. Demonic Sacrfice it.
4. Enjoy the BG with it.
5. When you come out your pet will still be summoned.
Soo you can use demonic sacrifice in every BG without wasting anymore SS.
Dunno if this is bug or not but I liked it and using it now ^^ .