Argent P/Sentenza don't gain much when they have a typing advantage or lose much when they're at a disadvantage because of their split type. Because guns do such low damage, you NEED that typing advantage bonus to be effective. The alchemer series are great for this purpose, providing the opportunity for type advantage as well as bouncing attacks and status effects. In terms of piercing weapons, you'll probably use either the magnus line or the autogun line, but these both prevent you from moving while shooting. Because piercing weapons are intended for use against wolvers and fiends (including vanaduke) it's common to use Argent P/Sentenza instead so that you can still move. In that case, pick between wolvers and fiends (hint: you want to kill fiends).

In terms of alchemer effect choice, volt or cryo drivers will help you kite enemies while doing good damage, and fire will do more damage but won't help kite and won't work in firestorm citadel. It's worth noting that fire/volt backfire against oilers/quicksilvers respectively, while cryo has only very slight backfire against frost slimes. I personally recommend volt because you won't have to worry about teammates breaking your freeze effect.