This sort of thing is pretty easy. I'm guessing your example is a trigger, so I would make a new trigger to look like:
(\w+) says\: "Iceshield."
From there, you make a variable from the regex wildcard by doing:
local person = matches[2]
(Notice that I used local. If you're only going to be using a variable in one place for one script, it's generally better to use a local variable, and not clog up the global variable namespace. From there, you'd just send the command:
send("cast iceshield on "..person)
Even though it's kind of a cluster, I'd really read through the Mudlet manual and absorb as much as you can. Afterwards, read through a Lua tutorial or something to learn the ins-and-outs of Lua. You'll be very thankful when you're done!