IME, direct damage is nearly useless in fights against single strong targets. Magic is great at AOE against lots of mooks, but there are almost no save-based single-target damage spells with focused damage – the only one that really comes close is sudden bolt from the Extinction Curse AP (2nd level, 60 ft, basic Reflex, 4d12 electrical damage, heightened +1 for +2d12 damage) and since nothing close has been released since I'm assuming they considered that to be overpowered. Pretty much every other single target damage spell uses attack rolls and/or has damage on par with AOE spells, and as a caster you should absolutely never use a spell with an attack roll against a higher-level target.
The tactics that have worked best for me as a primal sorcerer has been to use AOE damage against multiple opponents, but if we face a single tough one I use slow and/or fear, and possibly some buff on our main melee character if it's needed (e.g. fly to deal with flying creatures). At 11th level, using a 3rd level slot and two of my actions to deprive the boss of one action is probably a better use of my time than using a 5th level slot on cone of cold dealing an effective 25 points or so of damage (considering the significant chance of a save for half damage, and that's before resistances and such).
I've had great success with Flaming Sphere, Magic Missile, Sudden Bolt, Lightning Bolt, Fireball, Disintegrate (+True Strike), against single targets.
Take
Lightning Bolt for instance on a base level 5 Wizard that just got it, cast against a
Level 8 Chimera-- the base level 5 wizard has a spell dc of 21 and the Chimera has a +16 Reflex, meaning it succeeds on a 5. But, if it rolls anything below a 15, takes half damage-- reflex is its middle save. So it actually takes damage from the lightning bolt most of the time, increasing with frightened, which is a thing parties usually want to inflict.
Lightning Bolt does 4d12 damage, which averages to 24 damage, doubles to 48 damage if it crit fails (requires a 1 here), and 12 average damage if the target succeeds its saving throw, the chimera has 135 HP. If you get your 1/4th failure chance, and my 2 am math isn't mistaken you've done 17% of its health, if you get your 2/4 succeeded save you deal about 8.5% of it's health, with nothing happening on the remaining fourth. That's without the 1/20 odds of it rolling the 1 on its save and you dealing 34% of its health with one spell, or you rolling better on damage, or any modification from the frightened conditions (or any other conditions) that the whole party loves, and you have a leftover action-- my wizard used its evocation school force bolt to add extra damage.
Given your odds of dealing something every round and the fact that this is a solo boss, that's not bad if AOE is what you have prepped. Sudden bolt does even better (being a lower level single target version of lightning bolt with 1 per level scaling), since it's actually 5d12 at this level, averaging to 30 full damage and 15 half.
If we go the up cast Magic Missile route here (which I loved on my Wizard), your level 3 slots using all 3 actions are providing 6 darts, worth a total of 6d4+6 damage, which averages to 18 damage which is 13% of the Chimera's health every turn you do it, going up or down by chance. Considering your martial's numbers are balancing their hits and crits with complete whiffs, that's really not bad at all for a single turn for 1/4th to 1/6th of the party. If a combat lasts four rounds, and you keep it up, you'll have done 52% of the bosses HP
personally.