I'm currently using dual 10-iconers from NQII and honey potion as my bomb. Should my next upgrade be the 13-iconers or should I save up for ghostkerbomb for 1p? Thanks in advance!
I would skip the GBomb and go straight to SOS actually...
Of course 13-constants. Bomb is only Once per battle.
There are a couple of things to consider: useability, and price.
As far as price goes, you can (roughly) get dual-13-iconers OR Ghostkerbomb. 6 icons extra per turn versus 11ish more in one turn over Honey Potion. Here, dual-13-iconers win.
Useability: Against a frozen opponent, dual-13-iconers will mean 39 net icons against a frozen opponent. Ghostkerbomb (and a 10-iconer) would average 51 net icons (rounding down to 24 for Ghostkerbomb in this case). That's 12 net icons' difference in one turn. It would take two more turns during the fight for the dual-13-iconers to match this. Generally speaking, the dual-13-iconers win here, too, but there's one final bit to consider.
What type of One Player battling are you doing? Just DoN and messing around, or maxing out/plots? For the sake of singular wins, the Ghostkerbomb is the better option. For the sake of maxing-out, the dual-13-iconers are probably superior (requiring more luck/strategy: it depends largely on the rest of your set).
If you're willing to wait out the extra 3.5 million, Sword of Skardsen wouldn't have the weakness of the dual-13-iconers, and it would be faster at making up for "lost" damage (comparing to Ghostkerbomb). However, if you're a strictly One Player battler, it would probably be more worthwhile to put the extra 3.5 towards various full-blockers/reflectors.