Thanks.BTW, for me your OP hit the right balance between "I don't want to nerf him, just take him out of his comfort zone occasionally". I'm with that as well - let them enjoy their character, but toss things up.
Some good advice. They will be meeting some giants soon (there are some on dungeon level 14).They only have two ASI and aren't at the point where CHR adds to saves, so unless it's rolled ability scores they shouldn't have an amazing CON save.
Magic Missile is a wonderful way to blow concentration, especially if you have several junior casters doing it. A number of separate DC 10 checks, ignoring AC and absorb elements. Countered by Shield if they haven't used their reaction yet, but if they do then maybe it's time for the master caster to throw a big area of effect while Absorb Elements isn't handy.
Talking about that, my bet is their DEX save isn't high. Absorb Elements only protects against Acid, Cold, Fire, Lightning and Thunder. There are plenty of spells and monster breath weapons and the like that are save based and do other damage types like Poison.
Fliers could be out of range and make the target not worthwhile.
The bane of any caster is enough encounters between long rests. Though spell slots for a 10th level caster are pretty plentiful.
A bunch of giants or other heavy-hitting meatbags who can take the damage without worrying too much about it, have reach so they aren't clumped up to hit lots of them plus still attack with movement penalties, and if if they do go after him will hit hard enough to trigger a tough Concentration save.
A foe that grapples and moves him? Grapple doesn't go against AC, it goes against STR (Athletics) or DEX (Acrobatics), ignores the dodge condition, those reaction spells, and his massive AC.
Terrain and ranged attackers that make it hard to have a lot of foes in his Spiritual Guardians at the same time?
Again, don't do any of these frequently - this is just to occasionally need to adapt to being out of their comfort zone, not to regularly shut down his tactics.
Just one point: he's a dwarf, so targeting him with poison specifically is worse than targeting him with an element that can be resisted by absorb elements. (As a forge cleric, he's also resistant to fire.) I'll need to target him with radiant/necrotic/force/psychic.