While you're still getting 3 elements done with the Panlong, keep breeding those for Earth, Air and Water. So 21 more of those. That will leave you with:
27 Lightning
2 Earth
30 Plant
40 Cold
47 Metal
Now we're a bit stuck because Plant doesn't combine with Metal. Cold generally has the longest breed times so I'd combine Plant with it, making 30 Lichens to leave us with
27 Lightning
2 Earth
10 Cold
47 Metal
27 Magnetics, 2 Irons and 10 Mines will leave us with 8 Metal dragons to get. So,
21 Panlong
30 Lichen
27 Magnetic
10 Mine
8 Metal
2 Iron
Now because you have 8 x 1 element dragons (the Metals) you could split up 3 of those panlongs to their separate elements to combine with the metals to give you 3 Iron, 3 Chrome and 3 Rust. This will drop total dragons required by 2 and save you 3 x 36 hours of breed time (replacing each one instead with 14 hours for an Iron, Chrome and Rust dragon for a Net Gain of 22 hours). So the final tally is:
18 Panlong
30 Lichen
27 Magnetic
10 Mine
5 Iron
3 Chrome
3 Rust
96 dragons all up.