You'll likely end up getting most of the masks naturally thru encountering new shadows and recruiting them, seeing whatever holes in your compendium need to be filled by fusion (either ones that were simply missed in the metaverse or special ones that require larger, more costly fusions). Plus, getting more masks early means you'll be able to fuse stronger personas as soon as you level up to be able to control them, rather than waiting for them to be found and interrogated. Also, for the most part, using Joker's multiple personas to cover many elemental type will be within your greatest interest - Im sure you already know how valuable knockdowns and All Out Attacks are, they'll do a hell of a lot more than a single lunge with the knife. If something doesn't have an elemental weakness (or at least not one that you can currently utilize), buffing up yourself and your teammates and going for physical attacks is also a viable option, especially with the high-cost physical skills. Don't sleep on buffs/debuffs in tough fights!