The key thing is to use everything that would minimize you getting a bad streak of dice rolls (grenades, rockets, snipers, shotguns). Even then, I doubt anyone could have 100% completion rate on classic ironman. I'd say 80-90% is achievable though.
100% would be terrible design. At that point the game's just a flowchart and you're in danger of a situation where a page on GameFaqs allows someone to beat Classic Ironman the day they purchase the game simply by following a build-order and scanning down a list of 'if-then' conditions.
For Classic, I feel 80-90% might be too high. It serves the player who
plays expects to win, which I kind of feel isn't at the heart of XCOM.
I think of it like this: For any given outcome of a game (ie. did you WIN or LOSE), there's three primary emotional responses in the player -- disappointment, acceptance and elation. The key one there is acceptance and it does include the quality of 'happy' (just not fucking ecstatic). The key defining characteristic of acceptance in this sense is expectation, the expected outcome. I don't believe that games can necessarily achieve all three responses in a wider, general sense; typically, any given game's outcome emotion (determined by the personality of the player) resolves itself as one of the pairs, either disappointment/acceptance or acceptance/elation, obviously corresponding to lose/win.
Nearly all games run with disappointment/acceptance.
But, if a game has the stones to come right out and label itself as a difficult game, it should be managing the latter pair, acceptance/elation. And this falls as much on the player going into the experience as it does on the design of the game's challenge. *A player should
expect losses (accept losses!), regardless of their playtime/experience level, in any game that's pushed as a difficult one. Especially in the case of strategy games and certainly in the case of ones that use dice mechanics.
Going back to what I said earlier, '
For Classic, I feel 80-90% might be too high. It serves the player who plays expects to win, which I kind of feel isn't at the heart of XCOM', what I mean there about a player playing to win is that their expectation is a win condition; that's the acceptable outcome[. Classic should, I feel, always feel challenging. Which is to say, a loss should always be acceptable. And you should feel fucking great about yourself every time you win. Basically, after getting a solid grasp of the game's strategy, Classic should cater to acceptance/elation players, where Normal difficulty should cater to disappointment/acceptance ones. I feel 80-90% win rate would be ideal for Normal.
All this pontificating is assuming Ironman mode, by the way. Videogames' grand unifying time-rewind mechanic of save/reload nullifies these thoughts.
I'm ignoring Impossible difficulty here because I feel that that represents a more 'novelty' sort of experience, where you're playing simply to see if you happen to be the outlier sample who rolled nothing but double-sixes.
EDIT: * Added this part to the end of this paragraph, since it's a point I don't think I stressed.