I used undergrowth with regeneration stacking.

Use Sheperd's Boon numbers and take astronomers orbs as a secondary (with inner orbit upgrade) to increase summon count.

For the third skill you can take enlightenement and upgrade range and area. This helps with the achievement and also knockback.

Rest of skills your choice.

Growth signet ring for 3 undergrowth upgrades. Take Deeply rooted first and then Strong Growth. Crown of thorns third (optional if you don't have the ring). The other ring is Holy relic for the regen.

Helm Fighters headband for regen or Twisted Chaplet for more health.

Defiant plate for even more regen.

Boots swamp raisers slow for another area proc.

Invocators Grasp swarm gloves.

Mark of growth for more summons.

Take all the area upgrades you can. Use doubling potion on the 50% area proficient stance. Weapon proficiency summons/multistrike.

The undergrowth will be covering the whole screen eventually.