I believe Champion is somewhat misleading as a league starter recommendation, while Slayer offers significantly higher damage output.
To make fortification builds work effectively, you'll need costly unique items from trading, which won't be readily available or affordable early in the league. Additionally, you'll need to craft a chest piece with stun immunity because Unwavering Stance conflicts with Perseverance.
In my opinion, suggesting Champion outperforms Slayer for league start (especially in the first few days) isn't accurate.
A better approach would be starting with Slayer, then transitioning to Champion once you've acquired the high-value items needed to maximize its damage and defensive potential.
Just expect to invest a substantial amount of currency.