Everlast Tower

1F

The path to the east only leads outside so ignore it for now. Go upstairs.

2F

Head south to pick up a Turbo Ether. Next, go downstairs to the southeast to access the chest containing an Ancient Bow in the first floor. Backtrack to the second floor and take the stairs north (2F's middle-eastern stairs) this time. You'll be in the enclosed area in 3F that has a Kiku-Ichimonji at the end. Backtrack again to the second floor.

Head to the northwest this time and pick up the X-Potion from the chest along the way. The stairs in this floor's northwest and northeast corners are connected in 1F so you can take either of them. Since you're nearest to the northwest corner, take that stairs.

1F

From the northwest stairs, head to the east and go through the window to access a hidden passage leading to a chest containing a Rune Blade. Head southwest and take the westernmost corridor to find a Turbo Ether. There's also an Earth Drum near the northeastern stairs in this floor. Make your way to the stairs in the southwest corner then go up.

2F

Head to the east to find the anchorite of bolts, mouthpiece of Deus Ex.

Deus Ex

This is a lightning-based summon so equip one character with Thunder Charm or Abate Lightning (Summoner JLV 2) The latter is preferred since you can use the accessory slot for something else, like Force Armlets, Mage Shell or Barrier Shroud. Get his/her magic defense as high as you can and make sure you have high HP. Don't bother using the Reflect Ring since summon magic can't be blocked by Reflect.

After getting the summon, take the stairs to 3F.

3F

There's a locked blue chest in the middle-north of the floor and an Elixir in a chest in the dead-end to the south. To proceed, go upstairs using the one in the middle of the map.

4F

There are three chests in the area. The stairs in the middle is the correct way so ignore it until you collect all the treasure chests. To the west you can get a Dark Shield, to the southwest corner you'll get a Dragon Fang and the chest to the east has a Gold Hourglass. Once done looting them, go downstairs using the stairs in the middle.

You'll be back in 3F, continue to the next stairs to back to 4F again. Head all the way to the west and take the stairs to reach the temple's main floor.