First think about what the corporation is, and what about it you want to fight.
Explosives could be effective, but would be dangerous and risky and would be overkill and hurt a lot more than necessary, and the government would come after whoever they figured did it. AND, it would probably backfire because it would give corporations and governments more excuses and popular support for even more smothering and controlling laws, and could tend to reduce popular support and listening to anti-corporate messages.
Not that many corporations don’t deserve to be detonated, and it would be so satisfying.
Look at what the corporation is about, and whether it could be redirected rather than destroyed. Many corporations could be converted into harmless or beneficial organizations, if the people in control of them (executives, employees, stockholders, customers) could be shown that it makes sense and that they can get enough agreement to do so. See Speak Peace in a World of Conflict , and various Internet petition web sites (such as www.change.org) where you can sign and start petitions to corporations and governments to correct corporate policies.
The ”Speak Peace…” book I mentioned is about non-violent communication skills, but it also discusses and gives examples of how to get the attention, listening and agreement of people who can change corporate policy. The organization that publishes the book also has trained people who do that sort of work…
If a corporation has little or no potential for redemption, then finding sympathetic representatives in government, in the public (via petitions again, and in grass roots organizations), and possibly in lawyers and/or law enforcement or regulatory or better business organizations may be effective, depending on what mischief the corporation is up to, and who agrees that it’s bad and what to do about it. This might seem futile at first, but if you are pissed at what the corporation is doing, then chances are that you are not the first person to be upset, and that some people are already working on something to do about it.
If you find no people in your own country mobilized against the corporation you have in mind, you may find foreign or Internet groups who would be sympathetic and could bring pressure or legal action.
If there is a nasty corporation that is doing something like perpetrating copyright-based harassment of smaller companies, chances are that it is a puppet of larger corporations or others, who may be the ones that need changing.
Another option is to talk to reporters and bloggers and politicians and administrators about your specific case, and explain how outrageous it is and get it written and talked about.
Another option might be counter-harassment tactics of various types, depending upon the specifics of the corporation. If the corporation is really for-profit rather than a puppet, then anything that will hurt its monetary position can be effective. If it is a puppet, then tactics which result in it being a pain or embarrassment for its masters or government agencies, or that undermines and subverts its agenda, perhaps by resulting in an embarrassing media circus or legal case, can be effective. For example, Paul Watson of Sea Shepherd manages to operate an anti-whaling navy that attacks the whaling industry, and one tactic is to actually invite lawsuits, since he knows that it would generate a ton or media and sympathy and possibly a political and legal backlash that could be more effective (and do much less “collateral damage”) than a lot of TNT.