Another cantrip I was thinking about lately.  I like acid that eats armor.

Caustic Glob

Conjuration cantrip

Casting Time: 1 action
Range: 50 feet
Components: V, S
Duration: Instantaneous

You create a glob of acid in your hand and throw it at a target in range.  Make a ranged spell attack at a target.  On a hit, the target takes 1d4 acid damage, and if the target’s armor is not magical, the target’s armor takes a -1 penalty to AC.  This penalty is not cumulative with itself or other magic spells.  Additional spells that reduce the AC of a target apply the highest penalty to the target’s AC instead of the total penalty.

This spell’s damage increases by 1d4 when you reach 5th level (2d4), 11th level (3d4), and 17th level (4d4).

Thoughts?

I was thinking shield save you form a strike, but what about a spell that saves you from a crit?

Strike Break

1st-level abjuration

Casting Time: 1 reaction
Range: 30 feet
Components: V, S
Duration: instantaneous

An field of magic force stops a strike from decimating a target, but merely causes the target to be pummeled.  When you cast this spell, a target in range is no longer subject to a critical hit, but is struck as normal with an attack or spell.  This will only affect one strike even if the target is struck with multiple critical hits in a round.

Thoughts?

Let’s keep up with the against the giants push with another spell-Gravity Crush

Gravity Crush

3rd-level evocation

Casting Time: 1 action
Range: 150 feet
Components: V, S, M (a ball of lead)
Duration: Instantaneous

You cause the ground to instantly quintuple the gravity of a point in range.  Each creature in a 20-foot-radius of the point must make a Strength saving throw. A target takes 5d6 magic bludgeoning damage on a failed save and is knocked prone, or half as much damage and avoid being knocked prone on a successful one.  Creatures that are large or larger make this save with disadvantage.

At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.

Let’s keep building on the theme of big enemies taking bigger damage!

Power Throw

1st-level evocation

Casting Time: 1 action
Range: 30 feet
Components: V, S, M (powdered rock)
Duration: Instantaneous

You cause a large force to throw two targets into one another.  Choose two targets within range to make separate Strength saving throws.  On a failure, the target is throw up to the range of the spell.  If the target is throw into another target that also failed a saving throw, the recipient and and the target take magical bludgeoning damage as shown below based on their respective sizes.  As an example, if a large creature is thrown into a medium creature, both creatures take 2d6+2d8 magic bludgeoning damage.  The thrown creatures ends prone.  If no two targets fail their saving throws, then any who did may be moved up to the spell’s range and take damage as below as they are thrown to the ground.

At Higher Levels: When you cast this spell using a spell slot of 2rd level or higher, increase the number of targets of the spell by 1 for each slot level above 2nd.