you can use chapter select,you just have to beat everymission on fubar,it doesent matter if you skip,aslong as you beat all 15 chapters on fubar.
But honestly if you cant make it through chapter 4 then...you should forget it buddy.Im sorry but, chapter 13 urghhh,you will not be able to beat this mission if you cant pass chapter 4, trust me.
Chapter 13 is the real pain,is the hardest mission from all,you just can hope that you dont get grenade spam,but you WILL !its so annoying rahh^^.
Really , its not even worth the 50 Gs..I would never do it again lol.
But now tipps for you.
You have to kill a bunch of guys,and THEN lugo will tell you , " a grenade launcher,take cover" or something linke that,so if you kill the grenade launcher , it become pretty easy.But you need luck that the grenade launcher will miss you the first shoots.