It CAN be done at level 7, 6 even but you need Solas, Blackwall or Iron Bull and, Cassandra and all of you need to have full healing pots and full regen pots. Trigger the rift, make everyone take a regen pot, send warriors after the terrors, set solas on smaller wraiths, go after the despair demon yourself. Pause to re-up regen pots at 50% health, then let them eat health pots as needed after that. Also set all behaviors to 0% mana/stamina reserve and, be sure all abilities are enabled with Solas preferred on barrier so he keeps it up as much as possible and, one of the warriors set to defend you, while the rest follow you.
You won't just blaze it at that level, you have to work it, pause, pot, adjust attacks, hit a bit, rinse, repeat. it's a hard fight below level 9.
As for leveling, do side quests, search for codex entries, do any other rifts except that one and the one in the cave - it's nasty too.
By the way, if you get that rift closed at level 6 or 7, at level 10, go for the dragon - it's a 15 but, if you can take that rift, you can manage the dragon in Hinterlands or Crestwood at level 10. 