Boss monsters rarely are at 0 when you're using CR 20+ monsters.
The CR 10 Aboleth has +5, -1, +6, +8, +6, +4. The -1 is to damage dex spells. That's just CR 10
The CR 16 Planetar is at +7, +5, +12, +4, +11, +12
The CR 17 Death Knight is at +5, +6, +5, +1, +9, +10
The CR 19 Balor... +14...