How can I generate 250 GPa to create metallic hydrogen? In the preparation of metallic hydrogen, how does one conjure up such immense pressures?

I mean, I read that something like diamond anvils were used to contain the hydrogen, but now, in accordance with Newton's 3rd law, shouldn't the diamond also face the same extreme forces? 

I mean a 10 on the Mohs scale is very high, but how does that suffice to withstand the immense force applied back by the compressed gas?
According to The Compressive Strength of a Perfect Diamond 410-480 GPa is possible.  So, yes, the diamond experiences the applied pressure, but it is strong enough to sustain it. 

For more recent work, see Implementation of micro-ball nanodiamond anvils for high-pressure studies above 6 Mbar