For the endgame of Synergies mod you need 75% damage reduction. Once you get to level 100, you buy a scroll from the merchant Salan to travel to Table Mountain. There you can engage in challenging boss fights for legendary set items, farm gold in millions in the deserted gold mine, engage a variety of quite dangerous Trolls in Or'ak's Pass, and if you feel really brave, you can enter Darkness Falls, where each mob is a mini-boss.

Well it depends, you should divide the game into regular playthrough and end-game synergies content (wich is the real hard core of synergies)

Regular playthrough till reach the synergies end-content is still as vanilla, enemies level scale per zone. I wouldn´t say that renders previous areas useless, in fact, that encourage you to clean previous areas in order to be balanced for the next. Getting levels are supposed to reward you with making your character more powerful. If the enemies scale with you, gaining levels lose its utility. In that sense, Diablo 3 is somehow weird since it doesn´t matters what level u reach, enemies will be just same as you, so where is the point on getting levels, if it doesn´t really change the balance against enemies.

Now, on end content, you are supposed to be already on level 100, and thats just max. The enemies at this point are level 105 and above up to 200 or so, that means: At end-game enemies are "balanced" with you but not because they are the same level as you, but because they are balanced supposing the player has level 100.

At this point of the game you improve your character with dropping and crafting the synergies special armors, sets and gems, and also fame level (its much more harder to max fame than reaching level 100)

Okay that sounds pretty cool,
Yeah diablo 3 is all about getting to 'max' level 70. then end game content turns into all previous areas/random dungeons since all enemies are your level and drop the same type of items.
Anyway with the endgame items in TL2 in synergies, are the restricted by your stat points. One thing i found annoying in vanila was that i couldnt equip certain items because of it, i remember being them being like level 105 or a certain stat meaning you needed enough of that stat to equip it cause of the max level is 100.

Cool.