To get the achievement Its a me Mario , you just have to play at the Overgrown map and go to the machine and stay at the top , after that , just double jump across the picture of the woman and you'll hit a block with a (?) sign, and you will grow up just like Mario. Also after doing that you will unlock the Big Time mode and Small time too ( i think you just get the 2 modes after getting the achievement ) and you will be able to play with those modes every time you want .
To get the achievement Buns of Steel , you just have to go to any map ( i did on human campaing ) and play with allies and crouch behind your allies back for just some seconds and you will get it i didnt tried it on another map because i got it at Overgrown map by doing the same i said above but i think you can do it at any map .
i get the achievement buns of steel after get the achievement hide yo kids
People, to gain the I'ts achievement to me , skip this blue thing that makes jump higher , map Overgrown or Crash Site, under the woman after jump on it, go to the opposite side he takes , there will have something blocking the way , continue walking to the opposite side and will get it.