The mystical shield refers to a magical protection created by Eragon using his bond with Saphira. It surrounds them with a barrier that can deflect or absorb attacks, providing them with additional defense during battles.
eldest sister...
Ancient, aged, old, elderly, or decrepit. Those words mean eldest.
Mystical or magical princess
You get the Force Shield (one of the Mystical Weapons) from Sir Gawain on the Ice Planet (X-73 Y-83) where you use it to defeat the Tigercopter. (see related question)
eldest son
He ate his eldest child.
Shield
It depends on what shield you are referring to . Shield could mean a firewall or an antivirus like mcafee
"On [a/the] shield."
The "Waqa'at-i-Mustaqeem" was written in the 17th century by Dara Shukoh, a Mughal prince and eldest son of Emperor Shah Jahan. It is a mystical Persian work that delves into the teachings of Sufism and Hindu philosophy.
As Elemental, a great benefit is the ability to use a shield. Since there are quite some caster shields to choose from, you might go with the Devout Aurastone Hammer. If you prefer to not have to upgrade your shield, go with the Dignified Headmaster's Charge instead, which is a staff. Good shoulders are the Mystical Shoulders of the Elements, for the chest go with the Mystical Vest of Elements. Both trinket slots can be filled with a Discerning Eye of the Beast.
The term mystical can have several meanings. Its synonyms include mystics, mysterious, and occult. This shows that it means mysterious, and pertaining to mystical. It could also mean spiritual, magical, holy, and something beyond human understanding.