Can a synthesis summoner not wield a shield?

[Srtz]

Can a synthesis summoner not wield a shield?

[Artanthos]

He can, but it would not be very effective unless heavily enchanted.

The synthesist does not have shield proficiency and a shield won't stack with Shielded Meld.

[Imbicatus]

They are not proficient in shields as weapons or armor without feats or multiclassing, and will suffer ASF rolls when casting a spell while using a shield.

[Artanthos]

Assume a mithral light shield: no ASF or ACP.

You would need a +2 enhancement bonus to have any benefit at 4th and a +4 enhancement bonus to have any benefit at 12th level.

[Srtz]

say i multiclass into fighter (phalanx soilder) would he be proficient?

[Artanthos]

Are you going to sacrifice eidolon and spell advancement for 1 or 2 points to your AC?

You CAN do it, for a cost.

[Imbicatus]

Yes, if you multiclass, you would gain proficiency in all armor and shields, and in all martial weapons (shields are martial weapons). You would still suffer any ASF from the shield or armor heavier than light.

[Artanthos]

A summoner with proficiency can manage a mithral light shield and mithral breastplate without spell failure. The breastplate would provide no benefit while fused, but make a nice backup if anything happens to your eidolon.

[boring7]

Better to just use Extended Mage Armor and the shielded meld abilities, and spend your money on something else like Ring of Protection or Amulet of Natural Armor.