>>58084649
At 146 hitpoints, Lv 1 Spirit will recover 13 hit points to everyone
Lv 1 Guard will give yourself 20 hit points, but also bait attacks for your allies, which Zacian is particularly good at since it has a ton of resistances, and it's better at stacking, since Spirit can often waste hitpoints.
At Lv 1 alone, Spirit is probably better. But at Lv 2, we're comparing 40 hit points from Shield to 17 hit points from Spirit, so the gap widens enough to make Guard better, imo.

>>58016960
C Rank Dark Raid attacker, getting about 78% of Shadow Tyranitar
B Rank Flying Raid attacker, with 86% off Shadow Salamence (or 91% off Shadow Moltres if a double flying moveset is required)
You could do worse