Impish, Bold or Relaxed nature with 252 evs in defense .My bad I meant like how do I get my Steelix to its maximum defense of 548?
Basically, get the nature that raise the stats that you want : http://www.serebii.net/games/natures.shtml
Another example your adamant steelix at lv 100 with max ivs but no evs will reach 192 attack. Likewise, a max ivs and 252 evs in attack will reach 295 .
An impish,bold or relaxed onix will only reach 175 with no evs and 269 with 252 evs in attack .