Demon Temple is the eighth multi-player "dungeon," for level 65-70 characters.
When Wartune took away rage runes in June 2013, Demon Temple got a lot harder. You will need to have a rage set, either level 50 or level 60, before you can be effective against Azakar, the final boss. Without the rage set, mages can't keep up with healing and archers can't keep up with the scattershot required to defeat the final boss.
The easist team to defeat Azakar is 2 marges and 2 archers. If your party is strong enough to do a heck of a lot of damage per attack, you can do it with 2 mages, 1 archer, and 1 knight.
(The team depicted here failed. We were finally able to defeat Azakar with the 2 mages and and the archer shown here, with the assistance of a very strong archer in place of the knight. Squeak has the level 50 rage set. Shock has the 55 arena set. Squeak did fine with rage but Shock ran out as the battle wore on. I would have sent Shock home and found a rage mage to replace him, but the whole point of this run was to get Shock through normal Demon Temple so that he could support key runs in Demon Temple Nightmare.)
Get your MDEF and HP way high before tackling that boss. His MATK is about 48,000 hp with only one power boost. If everybody in your party has HP + MDEF > 55,000, you'll probably make it.
Archers should equip Arrow Shot, Deep Freeze, Scattershot, and Armor Piercer. Azakar powers up with every round so archers have to constantly debuff him with Scattershot; don't let him get above power level 2.
Mages should equip their full heal set: Restore, Suntoria, Blessed Light, Purify, and Lightning. Heal like crazy, whenever you can. Especially focus on using Restore because it cycles fast and most of Azakar's attacks are against a single player. If you try to do it only with Blessed Light, you will run out of rage and countdown time.
Mages also should watch out for the negative buff he lays on you. It stacks up, so use Purify to dispel it when it appears. It's the icon that looks kind of like a caduceus.
The Summoner in Demon Temple Nighmare is the mother lode of level 5 crystaloids and level 60 Legendary Stones. You will need both of these to build the level 60 Legendary set.
You only have to defeat the first sub-boss to get to Summoner, but he's as hard as the normal final boss. You can defeat him with the same basic approach as for the DT Normal final boss (see above) except that you'll need lots of PDEF for him. This guy doesn't do any magic attacks so if you have an option to trade your MDEF astral for better attack or PDEF, do it.
If you have a team available who can get you to the Nightmare summoner, you might as well go straight for the Legendary stuff. You'll get a good balance of crystaloids and Legendary stones. If not, try for the DT Normal summoner to get your crystaloids so you can make a level 60 rage set first.