Just done this myself, on hard, without anyone dying, and where I'd only just stepped into the entrance of L7 (so I wasn't too uber powerful).
Tried the "Throw everything through the portal" thing, but although I must have killed a couple, it didn't really do much and wasted tonnes of resources, so didn't bother with that afterwards.
Instead I gave my 2 backliners (mage and rogue) a healing potion each, 2 speed potions on my 2 fighters (human evasion, and minataur tank) at the front, and healing potions divided between my frontliners with the rest of my bottles. Everyone had shields. Rogue had the Poison shooting dagger, one of my fighters had the Flaming sword.
I popped my two speed potions and charged through the portal, IMMEDIATELY turned left, and started spamming fireballs, poison shots, axe swings and firesword shots at the enemies infront of me, took a few tries as my mage and rogue usually died too quick, but one time I got a lucky break where I only had two enemies between me and the wall.
So I charged forward, oblitterating those two (you can throw bombs at the same time just by picking them up and "dropping" them at the top of your screen, rather than using someone's hand cooldown). and got to the wall. Had to use a healing potion on everyone. Anyways then I clicked the secret button, immediately turned right, killed another one, and dived straight into the alcove and turned my back.
Then I was safe, my two fighters at the front taking ALL the damage. My mage then spammed fire bursts and spear pokes, my rogue merrily stabbing away with my two fighters using the other two speed potions, and then hacking away between chugging healing pots. While fighting I just kept making healing pots and slashing away, soon enough there were no enemies left, and tadaa, everyone was still alive.