I have a combined missile of 3 points fire and 2 points air. It works like hell!
Sneaking around, I can come close unnoticed, release the spell, and they drop dead immediately (level 8 guards are dead in one strike)
Such a spell does cost me 1/3 of my mana reserve though, so i need lots of mana potoins...
Splitting your elements like that is very inefficient, from what I've tested. Putting 2 in one element, and 3 in another element, creates a spell that does up to around 80 damage, but costs around 140 mana.
If you just put 5 into one element instead, you end up with a spell that does up to around 110 damage, and only costs around 80 mana.
Unless I am missing something, what you want to do, for maximum efficiency, is put 5 in one element, then save up 5 skill points, then dump those 5 (in one chunk) into another element etc. Otherwise you're just chewing through your mana unnecessarily.
BTW, a level 5 fire missile seems to do plenty of damage to a guard.