The learning curve on Nightblade is a little higher than other classes IMO because they lack a good damage shield or self-heal.

You can easily solve this by using 2-Hander. But DW + Bow is a bit harder because those lines lack shields or heals... I would suggest swapping Dual Wield for 2-hander while leveling.. or even go 2-hander DW.

To make things much easier for leveling, I suggest picking up 2-Hander tree. Cleave is the first ability you get and will morph to Brawler which gives a damage shield that scales on number of enemies hit. It pretty much makes leveling easy mode. The last skill in the tree is a self-heal that also provides a damage boost.

Also, a couple of tips:

Certain skills get big damage bonuses when attacking from stealth. This includes Ambush and Surprise attack. But also the Bow skill Snipe.

For groups of enemies, starting off with a Bombard (bow skill Arrow Spray morph) is a good starter as it does good damage and snares.

For AoE on DW, use Steel Tornado. The range for that morph is much better than the stam regen for the other morph.

Rending slashes in DW (first skill morph) can provide a self HoT, but it only procs on the bleed and is unreliable because the HoT stops if the mob dies or if the mob is immune (like undead) it won't proc at all.

Also, most NB skills are magicka based, while weapon lines are stamina based. So the more magicka + spell power, the more more your NB skills hits for, more stamina + weapon power means more damage from weapon lines. The exception is NB skills that morph to use stamina instead of magicka, when you morph them into stamina then they work like weapon skills.

So my final suggestion is if you are going weapon lines (as opposed to staffs) then stack Stamina and Weapon power and use the Stamina morphs. It's still okay to have a few magicka skills though so that you use that pool efficiently.