One is, the other is not. Which is which?
If we cut through the hyperbole and look at the definitions of these terms, we find that:
Decimate means ‘to kill every tenth person’
Annihilate means ‘to cause to cease to exist’.
One general election could not annihilate a party. In the case of the Liberal Democrats, this would have to involve its losing all its 57 seats at Westminster, all its 2,670 local councillors, all its 12 MEPs, 91 members of the House of Lords, its 65,000 members and its five members each of the Scottish Parliament and Welsh Assembly. However bad the result in 2015, one Westminster election could not result in all this.
However, the likelihood that the parliamentary party might be decimated at the 2015 election is a distinct possibility – a reduction from 57 to 51 seats would have the party performing above the position suggested by its current opinion polling ratings (although in 1997 under Paddy Ashdown's leadership the Lib Dems' share of the vote fell from 17.8% to 16.8%, but their number of seats more than doubled from 20 to 46).
The Liberals have been decimated before – in 1924, 1945, 1950, 1951 and in 1970. The Conservatives were decimated in 1923, 1945, 1964 and in 1997 and the Labour Party has had its share of decimations – 1931, 1970, 1979, 1983 and even in 2010. They all lived to tell the tale!