Yes, Harper has the constitutional right to ask for an election whenever he chooses, despite the horrible optics of ignoring his own LAW. However, rather than just assume the timing of an election is out of the opposition's control, there are tools available to derail Harper's quick vote plans. One angle, that would provide a powerful counter, the still lingering issue of election financing rules.
Currently, there is complete confusion as to what is legal and what is not. How can Canadians go to the polls, when we have a situation where the rules seem a matter of partisan interpretation? The Conservatives argue they did nothing wrong in the last election, "everybody does it", their legal case is "solid", which assumes that they could possibly invoke the same tactics in this election, so long as the current issues aren't resolved. We would enter any new election without the necessary clarity, so muddied that the notions of "fair" and "legal" seem subjective.
What I would propose, the opposition parties unite and issue a joint call for all parties to sit down with Elections Canada and agree on what is mutually acceptable. The NDP is already on record arguing that no election should take place until we have election law clarity. The Bloc, and Liberals, for obvious reasons, would both support the NDP position, which means you could have the spectacle of all three opposition parties singing from the same hymn book, all demanding that everybody understands the election rules, prior to any election call. In fact, it is easy to argue that an election call should be put in a holding pattern until these issues are resolved. In this way, you force the Conservatives hand, unless they are so arrogant to not see the political minefield of entering an election with the spectre of possible "cheating" on the table.
Imagine all the party leaders emerging from their farce of a meeting with Harper, telling reporters that they told the PM that an election now was impossible, given the current "crisis" surrounding election law. If one were to do this, it could be dismissed, but all three, then Harper is completely isolated and any quick move to an election is rife with negative perceptions.
The above is based on a principle, but it's more a tactic. Attempting to delay an election call a few weeks isn't a statement on "readiness" or "fear", but it's important to separate the bravado of "bring it on" with being shrewd. Everyone agrees, that it is too the opposition's advantage to get Harper back in Parliament, where they can pound him. The simple fact that Harper looks set to try and avoid Parliament's return, tells us everything we need to know about our strategy. With that in mind, attempting to delay an election, even if only for a few weeks, should be advantageous for all the opposition parties. Using the uncertainty about election financing rules is one possible avenue to undermine the Harper plan.