I have put Navigation on SOL Ed... that seems to do the trick on the Sphinx.
My team is as following:
SOL Ed : Navigation/Sustain/C Angel
Anubis : Yggs / C Serenity
Desert Warrior : Envious / C Serenity
Sacro : Alchemy / C Redemption
Sill the last wave isn't always successful and the team still gets killed ones in a while.
Plus 3* teams are just slow... the average is 5 minutes.