Based upon this test, it takes about 15 shots from sards to kill one guild tank.  This doesn't mean you need a group of 15 sards to kill a tank- a group of 5 sards each shooting 3 times will do the job fine.

Please note that all these examples are with non-promoted units.  I do not know how many sards or fremen would be needed to kill a lvl 3 guild tank. :(

3/7