Diablo – What are the parameters on the free seasonal sets

You are correct that level 60 is the current maximum level. However, as of patch 1.04, any experience earned after reaching level 60 contributes to your Paragon Levels.

Paragon Levels (Source)

• There are 100 paragon levels that a character can earn
• Each paragon level increases a character's Strength, Dexterity, Intelligence, and Vitality permanently and adds a permanent 3% bonus to both Magic Find and Gold Find
• The amount of time it takes to gain the first Paragon Level is close to the time needed to increase in level from 59 to 60.

Nephalem Valor

Another gameplay change that occurs at level 60 is Nephalem Valor. This doesn't permanently improve your character, but it does allow them to gain some bonuses to gold and magic find that last until they log out or change their abilities. See the link for a full description, but here's a summary:

Nephalem Valor is a buff that becomes active for level 60 players when killing an elite monster.

...When you kill some of the previously mentioned monsters, you get a stacking buff that increases both gold and magic find bonuses.

...if you change a skill, your Nephalem Valor bonus drops off.

Edit: As Holger pointed out, switching acts also causes you to lose the Nephalem Valor bonus.

Also, it isn't mentioned ion the site that I linked to, but there is a limit of five times that NV can stack. Killing more elites/champions will continue to refresh the bonus, but it can't stack more than five times.

The linked site also sounds uncertain as to whether changing equipment will reset the buff. I can confirm that changing equipment and/or potions does not reset the bonus.

Another benefit of Nephalem Valour is that each stack of it increases the amount of XP you gain from killing enemies by 15%. This improves the rate that characters gain Paragon Levels.

UPDATE

Paragon 2.0

Paragon 2.0 will still work as an end game feature, providing players with a way to continue earning levels and experience even once their characters have reached max level. There are several major changes to the system:

All experience earned by max level characters will count towards an account-wide Paragon level. This level, rather than the individual character levels, will award paragon points that can be spent on all characters; not just the specific character who earned the experience.

The Paragon levels earned and points awarded to Hardcore and Softcore characters are tracked separately. Since Paragon levels are for the account, Hardcore players will not lose Paragon levels or points even if/when their individual characters die.

Paragon points allow for a great deal of character customization, with specific bonuses that can be assigned to things like attributes, offensive stats including CC, CD, and AS, defensive stats like blocking %, and general properties like Faster Movement and Pickup Radius.

There is no maximum paragon level (though the individual bonuses may have varying hard caps).

The Paragon experience earned by all existing characters will be added into the system when it goes live.

Characters at Paragon 100 do not earn experience currently, so any play time with them prior to Paragon 2.0 is "wasted" in terms of exp gain.

Exp earned by all Hardcore characters on an account, dead or alive, will probably be added into the Paragon level when Paragon 2.0 goes live.

Many questions remain, and the system is still under development, but the basic functions have been confirmed by Blizzard as essentially set in stone.

• See more at: http://www.diablowiki.net/Paragon_2.0

I just read that sometimes if you use "summon zombie dogs" at any point during the session (some people claim even having on the bar) then it will bug and disqualify you from the achievement.

I just tested this, removed Summon Zombie dogs from my bar, left game, then resumed game. I just used "Circle of Life" to summon 3 dogs, one at a time, several minutes apart, and it gave me the achievement.

So it seems they do not have to be summoned "at the same time", just exist "at the same time".

The achievement does also appear to be quite buggy.

[edit] So, in term of what spells/runes qualify for this acheivement, sounds like:

Circle of Life, Mass Confusion with Devolution rune, Big Bad Voodoo with Boogie Man rune

And bugs which disqualify you:

There appears to be a bug with summoning them with the Summon spell at any point in the play session? Possibly with having the spell on your bar? Leaving and re-entering game fixes these, or at least did for me.