Use reimu's earth barrier on the first turn. Use alice's line shield spell and sanae's evade up buff for the remainder of the fight. Tenshi also hits with a party-wide petrification spell and hard hitting shield piercing strikes on random characters. use mokou as the commander with as many bombs as possible for quick resurrections and sanae's RES buff to counter these.
Use Reimu's elemental barrier on the first turn. Use Sanae to buff attack and defense spells as well as evade up buff. Use Dark elemental enchant on Youmu. Make sure Youmu is well-equipped with weapons that adds critical rate and is using Human Sword Stance skills. If you do this right, you can wipe out half of Tenshi's HP in just 4 turns or even less (if you can trigger Youmu's second attack in the same turn). Sakuya should act as a back up in case Reimu casted the elemental barrier at the wrong moment, that's where private square comes in handy.