This seems to kill everyone?

function onTouched(part)
    local h = part.Parent:findFirstChild("Humanoid")
    if part.Parent.Name ~= "Skelters" and part.Parent.Name ~= "Captain Stellus" and part.Parent.Name ~= "Lt. Brains" and h ~= nil then --Change this
    h.Health = 0

end
end
script.Parent.Touched:connect(onTouched)

== means 'is equal to', and you don't want h to equal nil, ~= means 'not equal to', so you want to say and h is not equal to nil.

Also I changed your 'or's to 'and's, it is just how I do it, I am not sure if or is proper.

Hope I helped :D

You just made a logic error. Say it out loud.

if part.Parent.Name ~= "Skelters" or part.Parent.Name ~= "Captain Stellus" or part.Parent.Name ~= "Lt. Brains"

To pass the condition, the name only has to "not equal" one of these strings. It's the same as saying this: "If I'm out of cereal OR if I'm out of milk, I'll go to the store." I don't need to be out of both; I only need to be out of one.

You're doing the same thing. "If the name isn't Skelters OR if the name isn't Captain Stellus, do this." Let's say the name is indeed Skelters. The condition will still pass, because it isn't Captain Stellus.

What you want to do is check if the name isn't Skelters AND it isn't Captain Stellus.

if part.Parent.Name ~= "Skelters" and part.Parent.Name ~= "Captain Stellus" and part.Parent.Name ~= "Lt. Brains"

I also don't know why you have h == nil. Perhaps you made a typo? You want to make sure the humanoid does exist.

if part.Parent.Name ~= "Skelters" and part.Parent.Name ~= "Captain Stellus" and part.Parent.Name ~= "Lt. Brains" and h ~= nil then

You're going in the right direction, just a couple of things wrong with your if-statement.

if part.Parent.Name ~= "Skelters" or part.Parent.Name ~= "Captain Stellus" or part.Parent.Name ~= "Lt. Brains" and h == nil then

What you've written, and what you're going for are two different things. First off, you want to make sure that you don't die if the character has ANY of those names, and so you shouldn't be using "or" between those, rather:

if part.Parent.Name ~= "Skelters" and part.Parent.Name ~= "Captain Stellus" and part.Parent.Name ~= "Lt. Brains"

That's what you're looking for.

As far as your "h==nil" is concerned, you have it backwards. You want to make sure that the humanoid IS present. Also, I would put that in an if-statement prior to looking for the names, so as to save your computer some time checking through the names.

if h ~= nil then
    if part.Parent.Name ~= "Skelters" and part.Parent.Name ~= "Captain Stellus" and         part.Parent.Name ~= "Lt. Brains"

As an aside, I would use a different method so as to keep your code clean in the future: Use an array alongside a loop to check for names, rather than just one large if-statement:

local Bosses = {"Skelters", "Captain Stellus", "Lt. Brain"};

script.Parent.Touched:connect(function(part)
local h = part.Parent:findFirstChild("Humanoid");
if(h) then
    local ItsABoss = false;
    for i, v in pairs(Bosses) do
        if(part.Parent.Name == v) then
            ItsABoss = true;
            break;
        end
    end
    if(ItsABoss == false) then
        h.Health = 0;
    end
end

This way, you can add bosses to the table "Bosses" easy-peasy.

Hope that helps!