You savescum till you get the sigil combo you want, its not worth playing "legit" as the best use of the sigil synthesis is fuse supp dmg V+ and those are hella rare, you really do not want to loose them.

It's a very lategame mechanics. Most sigils you'll eventually have surplus big enough that you can bruteforce good combinations just by farming Bahamut, Behemoth, Lucillus trio. Exceptions are war elemental+, Supp+, Berserkers echo+. Savescum those.
Damage cap+ become placeholder as soon as you can beat lucillus, because all 4 slots will be swapped for his unique sigils

Sigil fusion is really only useful SUPER late game if you want to really minmax a character.

The big thing sigil fusion allows you to do is ignore the rule that prevents sigils from having two of the same color of perk. Sigils cannot naturally have two offensive (IE: Orange) perks, but through fusion you can make sigils have whatever perks you want, regardless of color.

Wanna make Damage Cap + Crit damage? Go for it.
Double Tyranny? Sure.
Supp damage AND War elemental? Yep.

Though, RNG won't always give you want you want, so I suggest save scumming if you're trying to fuse anything really valuable. There's always the chance that the resulting sigil won't have ANY perks you want.

In other words, sigil fusion allows you to create perfect sigils with whatever two perks you want, regardless of perk color. You could for example make double damage cap sigils, thus allowing you to max out the trait by only using two slots.

You will either need a lot of fodder, or a lot of time to savescum the results though.

In case you're not aware save scumming in this game is simple.

All RNG based stuff (Sigil fusion, masteries, Transmarvels, redeeming vouchers, etc) is seeded when you enter town. So all you have to do is save before you do any rolls and if you don't get what you want, just reload your save and do a quest. Then after the quest is over you'll get different results. I believe the quest has to finish for the RNG to be shuffled. If you just enter a quest and quit I don't think it works.

TL:DR:
Save game > do any rolls > if nothing good > reload save > run any quest > try rolls again.

Yeah, I forgot about save scumming, haha. Now I gotta use that for the Transmarvels too, to try and get some character’s double sigils, which I haven’t even seen yet. Should have remembered sooner, but oh well, haha.

Originally posted by Xaelon:

Supp damage AND War elemental? Yep.

It would've been good if you could actually use WE in synthesis