Wired's Danger Room blog has a great post on how to kill movie giant 500ft movie monsters what weapons in todays military arsenal would be most effective. Every time I see a monster movie I think the same thing. For what its worth, I think any of the penetrating weapons such as the Massive Ordnance Penetrator should do the trick. Anything that can pass through 28ft of hardened concrete before detonating should give a monster a fair headache.
It's difficult to make accurate assumptions about 500 foot-tall fictional monsters whose very existence violates the laws of physics. But it's liable to have skin, scales or other outer with protective blubber or equivalent covering several feet thick. This will absorb anything except apart from an armor-piercing round. Flesh, like water, can stop virtually any projectile within a few feet – that's why you need something very exotic like a supercavitating round if you want to go through a lot of it. Those supercavitating Russian APS underwater assault rifles might be handy here... but you'd need a lot of rounds to have any effect.
Anti-tank weapons probably would not do much either. Shaped-charge HEAT warheads, like the one on the movie's Javelin missile, produce a narrow armor-piercing jet with relatively minor behind-armour effect. It might be enough to set off fuel or ammunition within an armored vehicle. But with against this monster, all you're just giving it a shallow stab with a hot needle: not dangerous, just very annoying.
There are some very good comments on the post as well.
Oh you are all nuts, you need to send another Monster to beat the first one up! Just hope your house dosen't get stepped on durring the boxing match.