General

Do every boss daily.

DIPQ, Commerci PQ, Voyage, Bossing usually works!

Bossing (i.e. Zakum, Chaos Zakum, HT, Chaos HT, Hilla, PB, etc.) especially during 2x gives great EXP.
I find Dimensional Invasion a good spot for solo training until 200 because of its insane mob spawn rate.

Yes, most of the time it is spamming Dark Imp and your hyper and for bossing the 1 mob skill. I did allot of HT and Dimension PQ and buy Enjoyable winter 2x buff it helps allot and get a mercedes to 120 easy with the burning event. Feel free to contact in-game on my main or Drk (TheRedDragon) and PB is also good. Warriors Ground at 190+

I love to have stable damage I also have easy 100% crit rate because of my main pha.

@believemenot: that really helps a dark knight.

use dawn warriors soul driver!

I trained at armory 2 until level 190 on my dawn warrior. If you don't have the damage to kill it, then go to hall of honor. Other than that, do daily bosses like everyone else said. Also, I've found that doing sudden mission quests can give a good amount of exp (if you aren't completing them while training). After you hit 190 just go to either mixed golems at warrior grounds or go to the scrapyard entrance.

As for your gear, it can always be improved. 300m will not get you too far, but you can get a lot of mesos or gear just off of daily bosses and doing gollux/commerci. From zakum, horntail, hilla, pink bean, easy magnus, etc, loot all of the equips to extract them for superior item crystals. Save the boss accessories if you need and try to aim for main stat. The ring from horntail, shoulder from magnus, and the face and eye accessory from zakum can give a pretty decent range increase. From gollux you can get hard carries from a guild and eventually get yourself a reinforced belt/pendant. Work your way towards getting a full reinforced set from the coins you get. With 2 daily farm runs and a hard gollux carry every day, it should only take you 10-12 days to get a full reinforced set. Also, buying the other gollux gears can make you a lot of meso. As for commerci, you can get cubic blades, cubes, or even sweetwater gear (if you are lucky) from the bosses. Saving up the denaros can get you the tattoo and glasses, which can be a good replacement for the zakum accessories. Also, you can do hard magnus with your guild every weekend to farm up coins to purchase the shoe and/or belt from the shop and get your cape from the boss. Tyrant gear gives a huge range increase. Try saving up your mesos to spend on a really good weapon/secondary. Use the cubes you get from bossing daily to try and get at least 6% stat on every piece of gear you have. I'm not sure how much damage it takes to kill the mobs that you are facing, but if you are not planning to get nx to upgrade your gear, then this is how I would go about upgrading my gear.

Bossing especially Pink Bean every day and during 2x. Then train on whatever you want until you can enter Warrior Grounds. Most of the good places have been nerfed, so bossing is the most profitable way to level for me.

When I had the same range as you, pink bean is like 15-20 minutes for about 30-40% exp. Just complete the time quests.
Also, you can go to the future maps.

Honestly not 100% where to train, but be sure to boss! Bossing gives pretty good exp, it turns out.

I use Spear Sweep to grab at things that are further away to reduce movement time, Piercing Drive when I know the attack will kill to reduce time spent in skill animation, La Mancha Spear to keydown for things like DIPQ and Commerci, Gungir when it's up for extra reach, and Nightshade Explosion is worked into my rotation around a map to hit multiple platforms.

Never forget that going onto a rope or ladder resets the flash jump count, meaning you can use this to flash jump multiple times if you get off a rope or ladder quickly enough.

Also remember that the skill hitbox seems to be at least very closely correlated to when the key is pressed - this means you can begin dropping down a platform, use impale, and drop down while still killing the mobs that were on the platform you're dropping from. Then, as you fall, you can kill something else if within range. Attacking by jumping or flash jumping off of ropes and ladders gives you more reach as well, and with multiple flash jumps, you can actually clear platforms in certain maps fairly quickly without having to actually walk on top of them.

Dark Knights have pretty limited skills, but just having flash jump alone opens up a ton of possibilities. Rush is useful too, for when I want to kill things in a spot and get somewhere else fast. The delay at the end prevents me from using it sometimes though; flash jump is much more fluid in comparison.