The Global Capital Market is big
The nation you are describing has good credit. If so, you may be underestimating the mass of the world's liquid capital. Unless that nation is the United States or Japan, then its total debt is not that significant on the scale of world capital.
Using data from the IMF's 2014 year-end report statistical appendix to get some perspective: the world GDP is about \$74 trillion, total equity and security market is is \$162 trillion, and total bank assets plus equities/securities is \$283 trillion. Because we live in an age of fiat money, where both bank and government can 'create' money by lending out money that they do not have, there is more money than there is produced value.
This is the important thing to know about debt: since money is a flexible concept, a country can (up to a certain limit) generate more money simply by lending it out from its central bank. And a bank can do the same thing; while individual banks do it on a smaller scale, taken together all banks in the world have lent over \$120 trillion dollars.
The amount of this money that changes hands every day is staggering. Some blogs suggest that \$14 trillion per day travel through the American financial system. The Bank for International Settlements (BIS) does over \$5 trillion in foreign exchange transactions per day between central banks, 88% of that to or from dollars.
The United Kingdom's sovereign debt amounts to \$2.06 trillion; France at \$2.44 trillion; Germany at \$2.42 trillion. Japan's (\$9 trillion) or the US's ($19 trillion) debt are significantly larger, while I'm not sure anyone knows how much China's debt is, if such a concept even exists for China. The point is that for all but the biggest two capitalist economies in the world, their total national debt is not even half of daily currency exchange flows.
From this data we can draw two conclusions. First, for the truly large countries (including UK, France, etc), you would need to be able to make $200 billion in payments to purchase around 10% of their debt. I don't see where you might conceivably get that kind of money, other than from a national government. That figure is about half of the annual revenue (not profit) of the biggest companies in the world, like Walmart. US total corporate profits were about 1.8 trillion, so you would need own about 1/5 of the US economy to get that much money in a year.
Second, for any country smaller, even if you were to sell \$200 billion dollars of some government's debt for a low price, in an attempt to prevent that country from borrowing more and thus wrecking their economy, that amount of money would be snapped up in a heartbeat by the existing global financial system. \$200 billion is a very large amount to buy as a single person/entity, but a relatively small amount compared to the vast scale of daily equity, debt, and bank transactions.