Even by upper krust standards CR 20 for that giant is way too hihg. I would rather take any mountain giant on than the possibility of a solar, titan, or a dragon of tha same CR. Its a no contest. It may do lots of damage, but a couple of spells nullify the threat as opposed to the other 3 where you will need to have a lot of luck on your side.
Maybe its just me but i always found it a problem since the beginning, that weak will saves and maybe fort saves susally kills you no matter what level. One of the biggest pet peeves i have against 3e. When a 1st lvl spell like glitterdust can affect a 10th lvl fighter with a fair chance of failing, i find it a problem. But then, thats just me.