Right, several things. You have a massive weakness to phasers, due to the majority of your team having to set up, and your revenge killer either dont hit very hard or are a bit slow.
Sigilyph is fairly gimmicky, if anything can hit hit quickly enough it doesnt last very long. On top of that it cant do much at all to dark types outside of psycho shift. Needs to set up and avoid phasers before it becomes very effective.
Medicham, IMO, is much better off choiced. It has neither the bulk nor the speed to set up effectively. I like a choice scarf set, using HJK+Psycho cut and a couple of elemental punches, but a choice band set hits incredibly hard, with HJK doing over 70% minimum
to physically defensive skarmory. Keep in mind that a resisted HJK does more damage than a neutral elemental punch. Reuniclus is nice, but nothing else on your team appreciates trick room.
Now, Gallade. The better of the two fighting set uppers, but still might be worn down too quickly to use salac with SD on there. You could opt for bulk up instead, if you want it to last longer, in which case you might want to put lefties on there. If you decide to keep SD and salac, get some speed EVs in there, you want to outspeed as much as possible. Life Orb/Lum Berry is often a better choice on an offensive SD set though.
Using Metagross as a scarfer without trick is questionable, but still viable. The problem with this one is not having any speed EVs. Take them out of HP and put them in speed. It absolutely needs these, base 70 speed just isnt anywhere near fast enough to get away without them. Also, choiced SR is generally a bad idea, it forces you to switch after only one turn. If you want to keep SR, put trick on there over EQ/Zen. So yeah, scarf with trick and SR, no SR, or no scarf.
Last but not least, Jirachi. Pretty solid, but sometimes not having protect on there with wish may come back to haunt you. You could use that over cosmic power if you want.
All in all, pretty solid team. You could squeeze starmie on there for coverage and spin support/revenger if you wanted, but that isnt really necessary. Other options could be wobbuffet for the countercoat trapping, Lati@s for a monster of a scarfer, and Zam for the magicsash. Good luck!