Merch Mini-Guide
Unique
SSB and mothership: pure damage (particularly raid), With the new raid rules you really only need one set for raid.
Lil princess: tanks with a skill-damage party buffer
Lorraine cush: tanks without a skill damage party buffer, use MP3 merch instead.
Mad Panda Trio: tanks and charge-in type heros generally better than lorraine cushion,
Coin bank: heal stat ba
Nari cushion: for... (not much, it works as a budget statue/invader ship if you dont have enough, but its generally worse as a dps merch, it does help kill twisted enemies slightly faster)
Lupina t-shirt: run-and-gun arena strats with melee heros, highly specialized
Ogh Model: for burst DPS attacking parties when you want to target/take out tanks very fast, good in colo attacking teams as it provides some defense as well.
Rare
Elemental merch: is a backup for any of the above roles as it does a little bit of everything, but is worse for any specific thing than unique merch.
Mayreel cushion: can be used for a lead hero in raid, it is usually better than elemental for lead raid heros, but worse than ssb/imr (but can fill the lead slot until you have enough sometimes)
iron teatan: is good in some coop expedition stages (anywhere proccing your weapon skill often and killing lots of small mobs is the name of the game) ok in tetis non-boss levels
all the normal merch is worthless.
Normal priority is SSB and/or Mad panda first.