My current character in Challenging difficulty. I haven't reached 20 yet but it is still fun to play with so decided to share.
I like to run dungeons without resting so having a wizard able to hit things using a blade on a stick (bardiche or glaive) is important for me. Also, it makes him less useless against enemies with very high spell resistance (e.g. Astradaemons).

The main idea was to take as few wizard levels as possible because it doesn't contribute to melee damage, but, at the same time, try to get some 9th-level spells. We cannot utilize Wizard School abilities to full extend anyway due to multiclassing. For this, I utilize Arcane Trickster because it gives caster levels and spells as wizard levels while improving melee damage by adding sneak attack modifier.

This build is good for using with reach weapons (6ft). Reach weapons are good because your character can stand behind other melee fighters in safety.

Final levels: Wizard 3, Vivisectionist 1, Arcane Trickster 6, Eldrich Knight 10.
This gives the same spells as 18 levels of Wizard + 5d6 sneak attack.
Also, we have Ranged Legerdemain, which helps with all those locked chests that only main character can unlock. It allows multiple tries in the game.

Starting ability scores
STR 13
DEX 14
CON 12
INT 16 + 2 racial
WIS 10
CHA 14

Dex is needed for AC (Armor Class) and to improve attacks using rays. CHA for persuasion by main character (you are a politician after all, you need to be persuasive). If you are building mercenary, you can dump CHA.

If you don't want to use reach weapons, it is better to use something with high crits like kukris or rapiers. In such case, it is probably better to move all points from STR to DEX.

Later we would improve stats using gear.

Levels distribution

Level 1: Wizard
Opposition schools are Divination and any of choice except Transmutation (it is important to have).
For Primary school, we wouldn't get many benefits because we don't take Wizard levels more than 3. I chosen Conjuration because it has good spells on every level (Snowball, pits, Stinking Cloud, Chains of Light, summons).

Divination is a good opposition school because there is not many spells that you have and they can be covered by your cleric or inquisitor (e.g. I got True Seeing from Harrim and See Invisibility, Communal from Amiri Huntmaster).

From spells, choose Grease, Shield, others by your choice. Avoid mage armor, magic missile and expeditious retreat because you would get them from scrolls anyway.

Feats differ if you are human or not.
For Human: Point-blank shot, Precise Shot (this allows to use Ranged Touch Attacks (rays) against enemies engaged in melee).
For Half-Elf: Skill focus Persuation, Point-Blank Shot.
Wizard bonus feat: Combat Casting (you would cast when being beaten up in melee at some moment).

Skills: mandatory: Mobility 1, Trickery 1, Knowledge(Arcane) 1 for Arcane Trickster. Persuation and Perception is a good choice for remainder.

It is better to take Arcane Bond — Object because it gives us extra use of highest level spells (which we have less than pure wizard).

Level 2: Vivisectionist.
It gives us 1 extra skill point, Sneak Attack (necessary for Arcane Trickster), mutagen and True Strike spells.

From alchemist spells, you would need to take True Strike and Cure Light Wounds because they are opposed or inaccessible for Wizard.
True Strike doesn't scale with levels and the most useful among divination spells so we remove biggest downside of taking Divination as opposition school.

Free feat from this class would help to kill spiders swarms in Bokken quest using bombs.

For Dexterity build, it is probably better to take Rogue or Knife Master for free Weapon Finesse.

Level 3: Wizard
Feat: Accomplished Sneak Attacker

Level 4: Wizard
At this point, you should have Mobility 4, Trickery 4, Knowledge(Arcane) 4.
Attribute point goes into STR to make it even.
This is the last level of Wizard we would take.

Level 5: Arcane Trickster
This should happen near the point when you conquer your capital from Stag Lord. You can now repeatedly try to unlock chests using Ranged Legerdemain.
At this level, it is no longer required to level up Mobility, Trickery and Knowledge(Arcane) because you don't need them to unlock other classes.

I focus on Persuasion, Perception, Knowledge and Use Magic Device from this point.
All skills should be your class skills at this point.

Feats: Martial Weapon Proficiency needed to unlock Eldrich Knight at level 7.
Level 6: Arcane Trickster
You unlock level 3 wizard spells at this level.
Level 7: Eldrich Knight
Feats: Dodge, Precise Shot or Skill Focus (Persuasion) + Dodge.
Spells like Scorching Ray become really good at this point.

Level 8: Eldrich Knight
Int +1 (19)
Level 9: Eldrich Knight
Feats: Combat Mobility
Level 10: Eldrich Knight
Level 11: Eldrich Knight
Feats: Arcane Strike, Outflank
At this point, the character becomes pretty good in melee, especially if using Belf of Physical Flow +4 belt (STR & DEX) and Extraplanar Cloak (+4 CON).
I don't understand why people say that Arcane Strike conflicts with EK Spell Critical since Spell Critical is active for 2 rounds after critical attack. It is important edge against enemies with high AC.

Level 12: Eldrich Knight
INT +1 (20)
Level 13: Eldrich Knight
Combat Reflexes
Level 14-16: Eldrich Knight 10
I got those levels together by doing unshared XP persuations in Armag tomb (persuade barbarians to kill Defaced Sister, then bluff to Zorek that you follow Gorum then intimidate him). It is easy if you put every level into Persuasion, take Skill Focus(Persuasion), use bard song and wear Mesmerizing Robe (my character has 44 Persuasion, enough for every check without rolls).

Feats: Spell Penetration, Weapon Focus (Bardiche).
Probably, taking Improved Critical (Ray) may be better but I decided to focus on utilizing Rod of Razors and melee crits.
I decided against Critical Focus because Harrim gives free criticals using his Aura of Destruction anyway.

I plan to put all remaining levels into Arcane Trickster for spells and extra sneak attacks.

Important spells:
1. Mage Armor at the beginning,
2. Shield,
3. Enlarge Person (to be far away from melee contact line),
4. True Strike from VIvisectionist (not much useful later),
5. Glitterdust (reduces enemy AC),
6. Blur,
7. Mirror Image,
8. Scorching Ray,
9. Slow (Haste can be put on the Bard because it doesn't need to have high AC),
10. Stinking Cloud,
11. Protection From Arrows, Communal,
12. Displacement (may be from a different character),
13. Greater Invisibility (for sneak attacks AND reducing enemy AC),
14. Echolocation (replacement for Blind Fight feat),
15. Stoneskin Communal (much better than the single-person version),
16. Icy Prison,
17. Chains of Light (good for controlling bosses and making the flat-footed),
18. Transformation (use when no spells left to become full fighter).
19. Frightful Aspect?

In the end, the character would have the Base Attack Bonus 14 (1 less than a Medium BAB class like Rogue), 2 Level 9 Wizard spells, 3 usages of True Strike per day, and sneak attacks.

If you want more Attack, you can make pure Dex build using Rogue instead of Vivisectionist and using Agile kukris and rapiers.
If you can live without level 9 spells, you can use 3 levels of Slayer to get more feats and BAB 16 which give you 4 attacks. It would also give you Martial Weapons Proficiency for free.

As for the gear, I use headband of Mental Perfection, Bracers of Armor (up to +8) and Robe of True Master (+5 deflection AC, frees ring slot from deflection ring). I wanted to go for Oak's Pelt first, but decided that I have too little number of feats to spend on Arcane Armor Training and the Robe gives same AC anyway.