Best way to build a sacrier??

For PvM water. PvP earth.

Why is the "air" element no longer used as it used to be in that version of the same pc?

It depends 100% on you, do you like cac, or medium distance.

CAC: Earth, Air, Water
Differences:
Earth is commonly used to tackle one character while doing area damage and stealing hp.
Air is commonly used to tackle multiple characters while focusing one character stealing less hp.
Water is commonly used to steal a lot of hp if you are surrounded by multiple enemies (Beautiful for pvm)

Medium distance: Fire.
Commonly used to deal a lot of damage and steal hp from a single objective, but consumes 4pa, anyways you can attack from a distance of 3.

Anyways its up to you, always ask this to yourself:
What do you like?
What do you want?
Why?

Then, take a decision. Good luck.

Another point of view for your decision is what you want to do once hit the 200 lvl. Eg. For Fri Dj the sacri sucks, it is useless. And first of all you have to decide what to specialize on, PVP or PVM. PVM chioice is rather limited (no FRI3). I am putting together CHA/AGI to make dopple  round rahter easy and farming ing (out of fri), but mine is just an approach. In other words I will park the sacri sooner or later levelling the second PG.

My personal opinion is that ANY Sacrier in DT should dual-element together with Agi. If you want your Sacrier to do the job that Sacriers were intended to do, you need that lock and dodge. Also, Assault with Air Damage stat is golden. Since it attacks twice, Air Damage and Damage are calculated twice. I would say the best thing to do is be Chance/Agi for start and mid-game for that PvM ability and switch to Str in the endgame because a Str Sac using a weapon on the 4th or 5th turn with Weapon Skill is broken damage. Excellent for PvP.