@darkwarriorkarg
Spell Combat does not work with dervish dance.
Dervish Dance:
When wielding a scimitar with one hand, you can use your Dexterity modifier instead of your Strength modifier on melee attack and damage rolls. You treat the scimitar as a one-handed piercing weapon for all feats and class abilities that require such a weapon (such as a duelist’s precise strike ability). The scimitar must be for a creature of your size. You cannot use this feat if you are carrying a weapon or shield in your off hand.
Spell Combat (Ex): At 1st level, a magus learns to cast spells and wield his weapons at the same time. This functions much like two-weapon fighting, but the off-hand weapon is a spell that is being cast. To use this ability, the magus must have one hand free (even if the spell being cast does not have somatic components), while wielding a light or one-handed melee weapon in the other hand. As a full-round action, he can make all of his attacks with his melee weapon at a –2 penalty and can also cast any spell from the magus spell list with a casting time of 1 standard action (any attack roll made as part of this spell also takes this penalty). If he casts this spell defensively, he can decide to take an additional penalty on his attack rolls, up to his Intelligence bonus, and add the same amount as a circumstance bonus on his concentration check. If the check fails, the spell is wasted, but the attacks still take the penalty. A magus can choose to cast the spell first or make the weapon attacks first, but if he
has more than one attack, he cannot cast the spell between weapon attacks.
Since both spell combat and deverish dance require your off hand to be free (this includes a buckler sheild, cetus, gauntlet, and etc that are straped to that hand but not thorn bracers that are strapped to your forearms and act as shields or weapons.
For all intetense and purposes your spell is counted as a weapon. Its the same reason you can not weild a two handed weapon. The off hand is not free and is supporting your weapon. So as a dervish dancer, he could use the scimatar to attack with as long as he is not using spell combat because his off hand is used to keep balance. He can not two heand the scimitar either for extra damage because that also removes the dervish dance ability. When he goes into spell combat mode, he loses his dex to hit and damage with the scimitar but can cast any spell that last 1 action or less.
The better weapon for a tank magus is a war trident because it has reach, is a one handed slashing weapon and is not considered a polearm because you do not get the standard -5 to hit with it in melee next to you at the cost of an exotic proficiency slot unlike a normal trident. It also meets the quantification for the black blade because it does peircing and slashing damage.
So from ten feet away your stabbing them, then casting in spellcombat without the fear of aoa or casting defenesive ((which gets ridiculously high at later levels on the concentration check at twice the spell level so that fighters have a chance at wailing in them and not getting vaporized into dust.