Have

Square Up

Target creature has base power and toughness 4/4 until end of turn.

Pest Summoning

Create two 1/1 black and green Pest creature tokens with "When this creature dies, you gain 1 life."

Swamp

Environmental Sciences

Search your library for a basic land card, reveal it, put it into your hand, then shuffle. You gain 2 life.

Daemogoth Titan

Whenever Daemogoth Titan attacks or blocks, sacrifice a creature.

Lisette, Dean of the Root

Whenever you gain life, you may pay {1}. If you do, put a +1/+1 counter on each creature you control and those creatures gain trample until end of turn.

Valentin, Dean of the Vein

Menace, lifelink
If a nontoken creature an opponent controls would die, exile it instead. When you do, you may pay {2}. If you do, create a 1/1 black and green Pest creature token with "When this creature dies, you gain 1 life."

Valentin, Dean of the Vein // Lisette, Dean of the Root

Forest

Honor Troll

Vigilance
If you would gain life, you gain that much life plus 1 instead.
Honor Troll gets +2/+1 as long as you have 25 or more life.

Access Tunnel

{T}: Add {C}.
{3}, {T}: Target creature with power 3 or less can't be blocked this turn.

Mage Hunters' Onslaught

Destroy target creature or planeswalker.
Whenever a creature blocks this turn, its controller loses 1 life.

Professor of Zoomancy

When Professor of Zoomancy enters, create a 1/1 black and green Pest creature token with "When this creature dies, you gain 1 life."

Witherbloom Apprentice

Magecraft — Whenever you cast or copy an instant or sorcery spell, each opponent loses 1 life and you gain 1 life.

Bookwurm

Trample
When Bookwurm enters, you gain 3 life and draw a card.
{2}{G}: Put Bookwurm from your graveyard into your library third from the top.

Search for Blex

Look at the top five cards of your library. You may put any number of them into your hand and the rest into your graveyard. You lose 3 life for each card you put into your hand this way.

Blex, Vexing Pest

Other Pests, Bats, Insects, Snakes, and Spiders you control get +1/+1.
When Blex, Vexing Pest dies, you gain 4 life.

Blex, Vexing Pest // Search for Blex

Dina, Soul Steeper

Whenever you gain life, each opponent loses 1 life.
{1}, Sacrifice another creature: Dina, Soul Steeper gets +X/+0 until end of turn, where X is the sacrificed creature's power.

Pest

When this creature dies, you gain 1 life.

Specter of the Fens

Flying
{5}{B}: Target opponent loses 2 life and you gain 2 life.

Tend the Pests

As an additional cost to cast this spell, sacrifice a creature.
Create X 1/1 black and green Pest creature tokens with "When this creature dies, you gain 1 life," where X is the sacrificed creature's power.

Fortifying Draught

You gain 2 life. Target creature gets +X/+X until end of turn, where X is the amount of life you gained this turn.

Mage Hunter

Whenever an opponent casts or copies an instant or sorcery spell, they lose 1 life.

Infuse with Vitality

Until end of turn, target creature gains deathtouch and "When this creature dies, return it to the battlefield tapped under its owner's control."
You gain 2 life.

Blood Researcher

Menace (This creature can't be blocked except by two or more creatures.)
Whenever you gain life, put a +1/+1 counter on Blood Researcher.

Witherbloom Pledgemage

Magecraft — Whenever you cast or copy an instant or sorcery spell, you gain 1 life.