Best method to get rid of mealybugs (and maybe scale too)?

I normally pick off the biggest insects (which are more likely to survive other treatments) using pointy tweezers or an alcohol-soaked sponge. Then I go through and apply a treatment. Insecticidal soap should work fine here as well, so long as it's mixed at a rate to kill mealybugs/scale and your country has reasonable consumer protection laws (if it doesn't, you may want to seek out an insecticidal soap that is marketed to commercial enterprises; just be aware that this may involve buying a concentrate at high expense and mixing your own spray).

If the infection is really bad, I prune off the most heavily infected sections of the plant. Then I remove the plant from its pot—including washing all potting medium off its roots, so the plant is completely naked. Then I basically give it a really through wash in the shower, hopefully knocking off any remaining large insects or loosely-held eggs, etc. Then I shake off the excess water, pop the roots in a container of water so they don't desiccate, and leave it until it's mostly dry. After that, I apply the treatment in the shower, with the plant still unpotted so that it's easier to thoroughly drench it from all directions. Once that has dried again, I repot it in a thoroughly clean pot with fresh potting medium. I also checked the area around where I was keeping the plant (mealybugs will scurry off and hide when you disturb the plant, and I have found them hiding in the tiniest indents in the windowsill and wall).

In my experience, these methods will really set the insect population back, but they may not completely eradicate it. These treatments kill insects only through direct contact (it may also need to be prolonged contact), so there may be survivors in various air pockets or spots you missed. Eggs in particular can often survive treatment. As such, you may have to do a number of follow-up treatments. I believe in the horticultural industry, they will typically re-spray every 3-4 weeks for the rest of the season to account for newly hatching eggs.

Even if you eradicate the population fully, you should keep an eye out with this particular plant hereafter, now that you know it's susceptible. These insects don't attack all plants; they only target plants that are too weak to defend themselves from pests. In a large grouping of plants, such as commercial greenhouses, it's common for only a tiny fraction of plants (with often BIG spaces between them) to get infected, while all the rest are completely insect-free. There are several reasons why a plant could be particularly vulnerable to insects: it might have bad genetics, it might have a disease that leaves it weaker, it might not be getting the care it requires for optimal health, or it may have diverted a lot of its energy to something other than self-defense (in my experience, plants that are about to bloom are particularly susceptible to pests).

I was advised that neem oil works on mealy bugs.

In addition to what you have already done, there is also the option of a systemic pesticide, which kills the insects that are feeding on your plant. This is a scorched earth sort of strategy, but it is also extremely effective. I use Bonide systemic granules on my indoor plants.

Do keep in mind that this should not be used on edible plants or outside plants.

You can check out food grade diatomaceous earth. Don’t forget to check underneath the plants leaves. https://www.tipsbulletin.com/how-to-use-diatomaceous-earth-for-mealybugs/