Paladin 6/Hexblade 5,6,7
Using Hunter's Mark, Hexblade's Curse and your Warlock slots for smites you are causing 10d8+2d6+24 damage (average 76) by level 11 with the duelling style and a long sword. It's only for two hits per short rest but you still have 8d8+2d6+24 for 3 more hits and then 4 more 1st level slots. I prefer +2AC from a shield to a bit more damage from a great weapon
You are fighting with Charisma from Warlock 1 so max out Charisma first, but I would start Paladin to have a chain shirt and shield for 18AC to start and not go Warlock until you have your extra attack
You only need 15 STR to wear the best heavy armour
So at level 6 you should be Paladin 5 Warlock 1, next level is paladin to gain aura of protection, thereafter Warlock to give you ever more powerful smites:
The 1st level of Hexblade gives you Charisma based attacks and the curse that allows you to add your proficiency bonus to damage to one target per rest. If they die you regain hit points. Also you crit on 19 & 20.
4th level slots at level 13 gives you 2 smites of a cursed target with Hunter's mark at 12d8+2d6+26 total damage (40-134 average 87)
For Pact Boons I would probably go Pact of the Chain to get a familiar to Help you - giving you advantage on one of the attacks. That is for targets without the Vow of Enmity. Being a tiefling I am imagining an imp or quasit so you can bait the Wizard's 2nd rate familiar. Pact of the Blade doesn't really help much as the Thirsting Blade invocation doesn't stack with extra attack and the Eldritch Smite isn't as good as the Paladin's. The Improved Pact Weapon would give you a +1 weapon but advantage is much better as is the scouting and other utility benefits of the familiar.
- Gift of the Ever-Living Ones to maximise healing you receive
- Relentless Hex -especially as a Mage Slayer
- Choice at 5th Level Warlock: Maddening Hex to cause 5 damage your hexed target and any creature you choose within 5'
Dex 16 or 10
Int 10[9 +1 racial]
Wis 16 or 10
Cha 20 [18 + 2 racial]
Choose between DEX and WIS for the possibility of better initiative and the defensive duelist feat as 1st ASI (use a rapier instead of a longsword - you still fight with Charisma) or just being more perceptive. I would choose Dex and let the rogue or ranger tell you when trouble is coming.
2nd ASI is Resilient for a CON boost and save, Sentinel (for reaction attacks) or Lucky. Lucky makes some misses hit, and reaction attacks from Sentinel increase DPR.
Other feat choices are Mobile, Mage Slayer (good with the aura of protection), Inspiring Leader, Alert, Shield Master. Alternatively, buy a mount and get Mounted Combatant.
Paladin 5, Hexblade 6, Fighter 2
This build does significantly more damage than the one above at level 13 with an Action Surged Booming or Green Flame Blade.
Building from scratch:
Paladin 1,2 to get Duelling, Smite and armour proficiencies.
Warlock 1, to be fighting with Charisma and to get bonus damage and crits on 19. Also to have Hex and the Green Flame Blade and/or Booming Blade cantrips.
Fighter 1,2 to get a second fighting style (Defence or Protection), Second Wind and Action Surge.
Paladin 3,4,5 for the ASI, more Smite slots and Extra Attack. The Oath of Devotion gives you Sacred Weapon as your Channel Divinity option which adds your Charisma a second time to each attack for a minute.
Hexblade 2,3,4,5,6 for the ASI, more invocations including Maddening Hex at Warlock 5 and the Accursed Spectre at Warlock 6
In terms of damage at level 13, assuming you have spent a round setting up your Channel Divinity(action), Hexblade's Curse(bonus) and Hex(bonus) with your first attacks, and you have hit(!) 4 times with action surge, you cause to your cursed target: (1d8+1d6(Hex)+5+2+5)×4(base) +4d8 x2(3rd lvl Smite) +3d8(2nd lvl Smite) +5(Maddening Hex);
to anyone you choose within 5': +5(Maddening Hex);
Finally have your spectre attack for +3d6
Total damage 18d8+7d6+53 = 78-239(av.158.5)
with 5 more to to anyone you choose within 5'
Assuming a couple of the targets allies within 5' over 4 rounds the average drops to 128 damage per round having used all your slots for smites. Hmm, if only you were a sorcerer too and could generate more...