Hey everyone, new here and wanted to chime in here.
First thing is the whole Iron legion, or in this movie the house part protocol. If you remember it was not till they were on the boat after saving the flight crew that jarvis told tony the debris had been cleared and the door to the iron legion could be opened, this is why it took till the end to see them arrive.
The reason the Mk 42 was weak was because it was still a prototype and not combat ready, hence him having to rip it apart to get to his missiles and such, Also it was a Prehensile suit. It was overly light at 20lbs designed to be able to work with anyone allowing it to be truly universal to who can use it. As a note the Mk VII from the Avengers end was 40lbs.
After the Avengers Tony spent his time not sleeping building suits for everything there was, by doing this he built an army but he neglected a true combat suit designed to withstand numerous attacks, The Mk VII was a true combat built suit with plenty of armor plating and at the end of the Avengers it was in excellent shape aside from the helmet and he had this outside the bar in the beginning. Had he focused on only 1-2 suits they would have been far more resilient but instead he focused on numerous suits designed for just one specific job.
When he ordered the clean slate and destroyed all his suits he told pepper he was going to trim down the herd pretty much hinting that he planned to only build a small number of suits, this will lend its self to more powerful suits. Also all of the suits in this movie had individual RT Cores just like Rhodeys suit did, same goes for the Mk 42, They chose to go the route of the RT Core in his chest was solely for keeping him alive while the suits had their own power source.