@Gamer. 1. That is a very fair and reasonable point. (I never complained about the railgun because their functioning is very simple. You just run a current through two parallel beams with your metal shell between them, and because of a quirk of magnetism, the right hand rule, the shell is pushed along the rails.)

2. That would make Blaze a super-charged torch, not an actual laser (Light Amplification and Stimulated Emission of Radiation). That is not a problem in itself, but it has some implications. Light Cores are used in facilities such as light bulbs, which emit light in all directions and of a wide range of wavelengths. The strength of a laser comes from the uniformity of the photons it produces, because the identical wavelength and same polarity of all the photons means that as a whole they have constructive interference (the wave amplitudes add up, making more power). In real life this uniformity is done by exciting gas atoms in a chamber, using electricity, not light. As is the nature of excited atoms, they only emit specific wavelengths. The chamber contains two parallel mirrors, and the photons bounce between them, getting absorbed then emitted by the gas atoms until very quickly they all become uniform. Photons going in a direction other than directly between the two mirrors are either absorbed or redirected, making the laser a straight beam. One of the two mirrors is only half-silvered, so some of the photons go through it instead of bouncing back. This is the beam, and beams produced this way are at present the most effective way to create high-intensity light. Filtering the light to create uniformity will be much less effective since instead of transforming photons you're just removing them. The cooling is necessary to prevent the two mirrors and the rest of the chamber from melting, and typically cooling requires twenty times more energy than the laser itself.

Given that light cores are magical, I will concede that if you found a high-quality light core you could make it emit uniform light, and given enough power it could be strong enough to perform your task. However, directing it into a single beam would still require something, like mirrors. Those mirrors, even if you don't have something that cuts through metal, would still need cooling to prevent them from deforming. Even if you could make it work exactly how you want, where the beam doesn't need to touch anything, radiant heat would still necessitate cooling. Your pseudo-laser isn't an impossible device, you just need to recognise that it will get hot as that is the nature of such devices.

3. I am aware that diamond-tipped tools are the strongest around. I am also aware that if something is travelling fast enough it will cause significant damage. However, you are using grains of diamond. These grains are small and have a minuscule mass, so even at 100km/h or higher they have a very small momentum. They have nothing behind them holding them in place to prevent them from simply bouncing off hard surfaces other than magic. Yes, you could cut through a neck with it, but metal or stone is a whole different story. Each diamond grain won't slice right through a rock or metal sheet- it would make a small chip and bounce off, exiting the beam of control and presenting a significant hazard to the user, or it could stick in the object being cut and likewise be lost to the machine. This means you'll quickly run out of diamond dust either way. High-speed streams of grains are used in sand-blasters, and they act like sandpaper, not saws. Really, it would probably be cheaper to use sand instead of diamond dust, as if you expend diamond dust every time you cut through something expenses would be very high, and sand is still hard. And before you say anything about re-collecting lost diamond dust, kinetic cores have no way of distinguishing between diamond dust and any other dust.

Of course, you could always say 'this is science-fiction so it works by advanced technology' and 'this is fantasy so it works by magic'. That explains away how a device can work when its functioning would otherwise be inconceivable. But it doesn't explain away how it overcomes practicality issues when its functioning is known. As a last resort, you could invoke 'suspension of disbelief'. It means unbelievable things remain, but it allows very interesting albeit impossible things to exist.