Ollin is a mage character that can be unlocked by getting below 50% cooldown reduction in a run before endless mode (and no redemption option allowed).

Ollin starts with the Mighty Mezcal Staff which fires a ball of giant cactus every 8 seconds. The cactus itself deals 2x damage and explodes into 10 smaller projectiles upon impact. The smaller projectiles deal normal damage themselves.

Interaction with Pierce/Bounce
If you have no pierce or bounce, the cactus would deal 12 times damage if it explodes on a boss (2x from cactus its and 10x from the 10 smaller projectile).

If you have pierce or bounce, the cactus would only deal 2x damage to the boss and then pierce through / bounce off the initial target and explode on subsequent targets.

Other Buffs
Ollin also starts with 20% cooldown reduction.

Drawback
His weapon has a 50% attack speed reduction penalty.
His ability does not scale with area of effect upgrade.

Ollin is a mage that benefits greatly from cooldown reduction and damage. His weapon has a 50% attack speed reduction penalty and hence he does not scale well with attack speed.

One point of pierce is recommended for wave clear. If you take too many pierce, it becomes harder to trigger the explosion. In my experience, bounce is not good with Ollin.

Critical Damage Threshold
Phase 1: 200 damage before 1st boss to one-shot T2 ranged units and dog skeletons.
Phase 2: 300 damage for T3 white hats and ranged units, 350 dmg for two-shot T3 zombie charger and 400 dmg for moles.

Core Items - Best 6 items before 1st Boss
1. Triple-shot
2. Back-shot
3. Far Sight (100% projectile speed)
4. Damage item (Gluttony or Eagle Eye, 60% critical chance)
5. Bullet storm (good for early defence)
6. Mezcal defiance / Onion / Bow gun (flex slot)

Items that works well with cooldown reduction

• Hero's Landing (requires some AOE for best result)
  
• Parting Gift (requires some AOE for best result)
  
• Plot Armour
  
• First Aid Kit

Items to Avoid

• Little friend (1/3 dmg & 4x attack speed) (we want to one-shot enemy with cactus)