If you have "Viridian Joiner" and "Umbral Mantle", you can create an easy, but strong, combo.
1) be sure the "Viridian Joiner" can be TAPped.
2) equip the "Umbral Mantle".
3) add 2 to your mana pool, then TAP the "Viridian Joiner" to add 1 forest to your mana pool.
4) spend the mana in your mana pool to untap "Viridian Joiner". It gets +2/+2.
5) TAP "Viridian Joiner" to add 3 forest to your mana pool.
6) spend the mana in your mana pool to untap "Viridian Joiner". It gets +2/+2.
7) continue to TAP and untap the "Viridian Joiner" in the way i just shoved to you.
8) you'll get a creature with infinite power and toughtness, and you get infinite green mana in your mana pool!!!