How to make a good Soren without copying Nino?

Your Soren will be a great magic duelist I think. His optimal build posted within the site makes him a pretty good unit. I have +speed on mine and he works wonders.

Build him like Sonya.

he is a very good Mage check and putting either Green or Blue Tome breaker to take down the speedy mages like Linde and Nino is a good option.
Having gronnraven and triangle adept makes him also great for checking colourless units.
Warding blow boosts his tanking capabilities against magic users.
WaterSweep and Darting blow (+Phantom spd seal) can be great for finishing of enemy units but he won,t be able to double which does hinder him.
Fury/Life and Death can also be a nice A skill as Soren wants to stay out of harms way most of the time unless the enemy is magic based unit so Soren benefits from Fury/Life and Death greatly
(Keep in mind that Life and Death lowers Sorens res.)

You're not "copying" Nino just because you're able to stand out in the same way she does (which to no surprise, is the reason why she's so good in the first place). That said, his best boon is by far Spd. +Spd Soren reaches 40 Spd with only Fury + Spd Seal or LD2.