Affected by a DC-boosted spell while casting.

[Serum]

Concentration wrote:

Spell: If you are affected by a spell while attempting to cast a spell of your own, you must make a concentration check or lose the spell you are casting. If the spell affecting you deals damage, the DC is 10 + the damage taken + the level of the spell you're casting.

If the spell interferes with you or distracts you in some other way, the DC is the spell's saving throw DC + the level of the spell you're casting. For a spell with no saving throw, it's the DC that the spell's saving throw would have if a save were allowed (10 + spell level + caster's ability score).

An opponent is currently casting summon monster IV. I cast waves of fatigue affecting her. I have a casting stat of 18, Spell Focus (necromancy), and am a gnome with the Fell Magic racial trait. What is the DC of the concentration check the opponent must make?

[Create Mr. Pitt]

10 + 5 + 4 + 2 = 21 + 4 = 25

[GM Rednal]

That should be, what, two above standard (Fell Magic and Spell Focus)? Pretty sure your DC there would be 21.

(10 Base, +5 from Spell Level, +4 from surprisingly low Int score at that level, +1 from Fell Magic, +1 from Spell Focus)

[Serum]

So, it's apparent that the text after what I bolded is just reminder text as to how a spell DC is normally determined? Although spells with no saving throw normally do not benefit from effects that add to the DC of spells, they should in this case?

[GM Rednal]

The part you bolded is the general rule for calculating a saving throw. Individual abilities like Spell Focus are specific rules that change the general rule (by adding on to it), and so they take priority. In your case, if a save were allowed for a Necromancy spell, then yes, DC-boosting effects would apply to it.