Though you could also play Master Duel on PC with BlueStacks and run multiple instances, which will easily let you keep multiple decks without the money or time investment The main methods of obtaining gems for free are by completing missions, both limited and unlimited, as well as through rewards from completing solo chapters
Trace the curved design on the front/back, side and center pieces
Some might even throw this term around for sets released in the last 5ish years
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On its own, mono-blue can get you creatures with combat damage triggers that draw cards (Neurok Commando, Tandem Lookout, Scroll Thief) and ninjas with ninjutsu and other combat damage triggers (Mistblade Shinobi, Mist-Syndicate Naga)
Untapping Lattice-Blade Mantis as its last ability resolves doesn't remove it from combat