Use 1 ES5 at a time, that gives you 24 tries. That's how people get lucky, they try many times and eventually success comes. Using 3 ES5 may combine the rates of 3 tries, but it's still a low rate, and raises the stakes since you have more to lose. Having 3 tries instead of 1 per try gives a better chance, since it's all about luck, numbers barely mean anything. I've failed twice in a row at 89%, so with enough divines and ES5 you're bound to succeed.
This really is the best way, since each new level gives a lower rate, and the leauge boost does less and less help to the rate (example: 50% leauge boost only gives 5% to the 10% lv.4 synths). This means even if you used 3 ES5 and 25% amulet, the rate would still be low since it would make a 15% (50% LB) a 17% at best.
It takes a while to get the required medals, it's something to work at everyday. You can also metallurgy ES3 at 89% for a ES4, but it relies on luck as well.
Everyday you can make 14 medals from LF/FF:
Morale Boost (2 medals) - Win sports fights, twice daily
Technique (1 medal) - Complete sports fights, twice daily
The Art of Combat (1 medal) - Defeat players, twice daily
Victorious Fanfare (2 medals) - Win league fights, twice daily
Note: if you win leauge fights before lv. 20 you only get 1 medal instead of 2 for this.
League Honor (1 medal) - Complete league fights, twice daily
Depending on what level you are you can get 4 medals from completing the different additional scenes:
Once you are above the max level (eg. 18 for ant easy) you cannot do the daily task anymore).
Ant Cave (Easy) 4 medals, Lv. 9 - 17
Ant Cave (Normal) 4 medals, Lv. 10 - 20
Gulu Kingdom (Easy) 4 medals, Lv. 16 - 25
Gulu Kingdom (Normal) 4 medals, Lv. 18 - 30
Gulu Kingdom (Difficult) 4 medals, Lv. 21 - 40
Gulu Kingdom (Hero) 4 medals, Lv. 26 - 40
Evil Tribe (Normal) 4 medals, Lv. 18 - 25
Evil Tribe (Difficult) 6 medals, Lv. 22 - 40
Evil Tribe (Hero) 8 medals, Lv.24 - 40
Dark Castle (Normal) 3 medals, Lv.18 - 40
Dark Castle (Difficult) 5 medals, Lv.26 - 40
Dark Castle (Heroic) 6 medals, Lv. 24 - 40
There's lots of medals to be gained from side/main tasks starting out, or leveling up. You can also get medals from seasonal tasks, packs and vanity.