12

Use a Covern or Kihunter. A flying Kihunter takes about 25 seconds to get to the door.

Requires:

{
  "or": [
    {
      "enemyDamage": {
        "enemy": "Kihunter (yellow)",
        "type": "contact",
        "hits": 1
      }
    },
    {
      "and": [
        {
          "enemyDamage": {
            "enemy": "Covern",
            "type": "contact",
            "hits": 1
          }
        },
        {
          "not": "f_DefeatedPhantoon"
        },
        "canRiskPermanentLossOfAccess"
      ]
    }
  ]
}

Dev note: The Kihunter will do more damage than the Covern, so whether Phantoon is dead or not, that strat will be accurate or conservative.