Whoa, whoa, whoa, slow down. That is way too many questions too fast. Let me try to answer them one by one.
Remain aware that neither the character of Sans nor Fresh was created by me, mine is merely a fan comic. The rules may actually be different, this is just my interpretation. Sans was created by Toby Fox and Fresh was created by Crayon Queen. (Just a couple names to put into your YouTube search engine if I'm not clear enough.)
To unlock the Sans boss battle in Undertale in the first place, the player needs to kill every single monster in the game prior, rather than sparing. Sans won't take you seriously until he finds out what you've been up to, at which point he decides to take action and stop the genocide megalovania.
San's blasters look like giant dragon skulls and fire giant beams of disintegrating energy. They're nicknamed "Gaster Blasters" because (in most fanfics at least) his last name is Gaster.
Sans is a skeleton and can thus use Bone Magic. He can summon bones and throw them at you like javelins or use them as spikes he can shove through the ground at your feet. I forgot to mention this, but he can also throw you around with telekinesis, slamming you into walls or more bone spikes. My character of Sans also wields two halves of a precisely broken bone as daggers!
Karmic Retribution. That's a tricky topic. Sans' attacks only do one damage initially, but they inflict you with more poison-like damage over time. That extra damage is the power of Karmic Retribution. Since it only appears in the genocide route final boss battle with Sans, it's hard to say exactly what the specifics are, but since the ability is called Karmic Retribution, it's safe to say the more pain you inflict on others, the more pain Sans' attacks inflict on you.
It doesn't matter how hard or fast you attack, Sans is always going to dodge you. He is small and light on his feet, and in extreme cases, he can also teleport. There is only one way to beat him - wear him out and then attack in his sleep. Only killers play dirty.
Now, Classic Sans can be killed, but Fresh is a variant from another dimension. Different things have happened in his life. One extreme example is the fact he has been infected with a truly bizarre parasite. It displaced his soul - source of life energy, in this case - from its proper place. I don't really know how it works exactly, but long story short, if you stab him, the worst that will happen is you ruin his hoodie. Any physical damage he takes will be regenerated very quickly, and even if you do hit his soul, nothing seems to happen. This is not saying he can't feel pain, though!
Hopefully this answered some questions. I would still look up "Sans fight Undertale" and then "Fresh Sans origin" on YouTube for some more answers though. Those videos can show you how hard this fight would be better than I could! I'm going to get back to work now.
