General

I had the same problem as dual blade when I hit 140. Even stopped training for weeks just because it was so difficult. What I ended up doing was buying auto hp. While it did drain a lot of potions and mesos, I can say it was worth it in the end! And life became easier once I past 165.

Set effects, Aran card, cannoneer/kaiser link, RA runds, scrolling with spell traces gives health bonuses along with luk, etc.

Get character cards. 3 decks of S rank would give u 2.1k hp.
Link skill from kaiser = +15% hp, cannon shooter = +10%.
Spearman Character Cards: S rank = 4%
HB pants.
Reinforced set. +2.05k +10% hp
CRA set. +1k, + 10%.
hp boost potions in fm or make them though alchemy.
Also, zero link helps cuz it decreases dmg.

Pets help survival by healing you, but dont rely on them too much.

No wait, level 140 di mobs are wimpy and hit like 1k. Just how low is your HP?

Also, you don't have to train it. You can just mess around on it at weaker mobs, although if you're the type of person who needs a goal to play a character, I understand why this wouldn't be an option.

Link skills and character cards are crazy helpful. Also since they made it cheap, why not invest in a pet+pet skills? I never noticed any squishiness because of auto HP (+daily RA runs which replenishes pot supply and more)
Drk card in particular gives 4% hp at S rank. Not sure if too insignificant or just forgot cus I don't want to repeat the other posters.

At 140, you probably dont need much funding yet and are just introduced to empress set. Dont go crazy trying to buy all the empress as it is better to get the royal 4 piece set later at 160s.

First get a Perm pet with HP pot buff
Get hp% link skill from Cannoneer, Kaiser.
Get 4 sets of cheap empress for that %hp
Get in with 2 ppl party.

Unfortunately they are squishy even at later levels and avoids do not work well like they used to.
It may be important to get a HB neb and put it in a royal wanderer pants later on.

Get full empress for %hp.
Kaiser link skill
cannoneer link
beast tamer...? I think that gives %hp but I could be wrong.
Perhaps also try to get the solid gollux set. I didn't look at the set effect in a long time but I believe it also gives some %hp
Hyper body neb/potential.
If you really need, buy some superior fortitude potions or something. They give +5k hp for 2 hours. I use those personally when I go to empress cuz my HP is so low.

I believe there is a significant HP and defense bonus by having 3 warrior type character cards lined up... that or 3 adventurers.

Also, you're a ranged class - half of your "survivability" is to keep away from mobs and use your range as your shield. You don't need to be close to your targets... so don't be.

I ran into the same problem at lvl 140, but as soon as you get your CRA/Gollux set, your hp should almost double. Along with some good %luk for avoid, that should be enough to keep a 10 second interval between pots, also a pet and auto-pot would be nice, makes it so much easier.

Get 3 S-Class Character Card sets, which give about 800 hp each
Gollux and Emp/Faf for the set effect
Cannoneer and Kaiser link skills (also BT if it comes out again)
Decent HB
Willpower Trait (2k when maxed)
If you really need it, cZak helm and Rex's earrings give 10% hp each

I'm sitting at 30k hp, and rarely take damage at SDH with 10k avoid and maxed shadow shifter. Before that, though, I used to get annihilated at Evo (pre-RED, when i had to stand next to mobs to use shade splitter) to the point that i did DRPQ to train.

If you really enjoy your Night Lord, then invest in a pet with auto HP! It helps out so much!

I would recommend filling your character card slots for the bonus HP!

Like others said, try getting the HP boosting link skills and go for sets with HP bonuses in them, such as Reinforced/Superior Gollux and the RA set once you hit 150. If you can't afford those, then go for Cracked/Solid and Empress!

I'm only at 20,000 health, and I find it to be more than enough for just about anything. Try to avoid taking damage, if you can.

Also, max shadow shifter for that extra boost of avoid, if you haven't already!

kaiser link, and get an emp set for it
if you plan on playing it often, it'd be worthwhile to get a pet and auto hp
if you're doing evo stand under the generator and jump and use showdown, you burn much less pots like this