Ingame walls are not really needed and I hate to break it to you but them Pokemon are not walls. These two examples are walls though.
So Joke over a wall is a Pokemon like Shuckle that usually has monster Defences or nice Hp.
Shuckle is a prime example as it has 230 base Def and SDef.
Ponyta on the other hand has 50/55/65 base defensive stats.... That makes it one of the worst walls you can have though if you persist on using it as a wall try ev training it in Def and Sdef therefor leveling it up as a lv 39 will fail in the E4. It can pull off a physical wall I guess as it has access to Wiolowisp (which cuts the foes attack in half) and it has moonlight for recovery. But if I remember a few of the Pokemon in the E4 have guts so becarefull.
Now unlike Ponyta Jirachi has nice 100/100/100 defensive stats and nice typing so can pull off a nice special defensive set. Though your current set isn't a walling set. I do not beleive you understood the role of a wall. I'm not going to suggest a set as that Jirachi will be walling already at the level it is and if you made it more physical it would be a great sweeper.
All in all they are not good walls and with the natures/stats never will be. But for in game walls slow down play and are just not needed. So I suggest making them sweepers. That will make the E4 much easier. As well as the game more fun.