You need stuff in beforehand so that you can fully commit to your killing spree.
You need to:

You need to get atleast 15 Scorpions to get Gold on that Side-Job.

With Gold, we now unlock the real deal of this Guide.

Mission 18 gives you 1,500 LB worth for every enemy you kill.
The Plan is simple, we bump the number of kills atleast to over 100 (~120)
But for the very first grinding we can't already use the fun stuff to make it easier

A very vital part of this thing to work is the Tsubaki Mk-III cause of it's chargeable special attack that shoots Travis through the air.
This kind of fun cost a lot of Battery Charge so try to upgrade it to its fullest capacity till you unlock the upgrade that makes you lose no charge.

For the start you're gonna use up your battery real fast with each charge attack, so you won't get as close to the 100 as you would wish, but this doesn't matter cause of the still high amount of LB we get, we can get progressively better stuff and make the grind a bit easier.

The Late Grind happens when all requirements have met

• Getting Gold Rank at the Scorpion Minigame

• Getting the Tsubaki Mk-III

• Getting the upgrade for the beam katana so it won't run out of battery so fast (or at all)

When this is all set you can get to the good stuff.

With all the parts in place you can now get to the killing spree.
Rundown

• Choose Mission 18

• Run or Drive to Gold Town

(Note: a positive about this way of using this mission to earn money is that you can switch between K-Entertainment and Gold Town very fast)

• Start the Mission

So, now i try to explain how combat works the most effective way in Gold Town.
Try from the moment the countdown lets you control Travis to charge the Beam Katana with the X/Square Button and letting of the Button go while mashing the X/Square Button repeatedly.
This results in this blurry mess:

Try to seperate the purple suits from the brown shirt punk looking guys] that you always clean half of the battlefield so that while the other half respawns you can already charge your attack again.

Issa lotta money

Now without the upgrades this seems to suck at first cause you have to keep recharging the Katana, this kind of sucks so I would try to get the Energy Saving Parts as fast as possible.

Now the real grinding has begun with starting the mission, getting paid and running back to K-Entertainment, repeat.

Have a good one.