I would never use the oven personally. You cant really keep an eye on them properly and there is so much risk and damage that can be done. Whereas with a heat gun (or ive even used a hairdryer a few times) you have complete control and there isnt much chance of damaging the lights unless you put it at a stupid temperature or really dont pay attention.
My key piece of advice would be to go into the job expecting it to be a slow process. Therefore you wont get impatient and frustrated, which can lead to trying to rush it and damaging your lights.
I just kept heating up a section, then pulling apart a little (starting with wedging in a screwdriver), then heat a bit more, then pull apart a bit more etc. and just keep going until they come apart. I had my heat gun at about 220 degrees, and i would say it takes like 30-45 mins per headlight to take apart?
Once they are apart its very easy as its just a few screws to take apart the internals, paint, and then reassemble them.
With regards to putting the lights back together, i bought plenty of sealant and put it all around the lights where the old sealant was. Dont use it sparingly, as you want to make sure the unit is well sealed and has no gaps that could cause condensation.
Once the sealant was on, i then went around the lights with the heat gun to soften the plastic again, and then pushed the lense back on. Then you can get the screws back in on the back to really pull the headlights back together.
It honestly isnt difficult to do, it just takes A lot of patience.