Image: Rebel Wolves / Bandai NamcoOfficial screenshot from Rebel Wolves / Bandai Namco.
Miser is earned by holding 5,000 denarii at once. It is easy to get if you stop spending for a while, and it lines up with saving for the Arbiter armour.
Build your purse up to 5,000 denarii. The trophy pops the moment you hit the total.
Selling silver in Svartrau helps and counts toward Smuggler at the same time. The Silk Threads for the Arbiter armour cost 6,500 coins, so saving for them gets you there anyway.