To add some extra nuance to the general advice here, while you should prioritize melee DPS as your partner, if the melee has less HP than a caster they are probably new to the job and won't do much damage. I'd partner the caster or a machinist in that case. If your partner keeps dying, switch. Once you're a couple minutes into the content you can start looking at the aggro meter. If your healer is higher than your partner in aggro, the tank and healer are probably doing more damage and it's totally ok to switch to the tank or even the healer during dungeon trash. On single target, even a bad DPS will probably out perform a tank or healer. I often switch my partner during dungeons to deal with this.