Magma Hellion — ruling 1

You may reveal a card with assist and discuss how to pay for it before you begin to cast the spell. Targets are chosen for that spell before you choose another player to help you pay for it and before that player has committed any mana to doing so.

Magma Hellion — ruling 2

Only the generic mana portion of a spell’s cost can be paid with assist. Any colored mana costs must be paid by the spell’s controller, even if the assisting player paid mana of that color.

Magma Hellion — ruling 3

If an effect allows a player to cast a spell spending mana “as though it were mana of any color” or “of any type,” that player must still pay for the colored mana in that spell’s total cost. That cost doesn’t become generic.

Magma Hellion — ruling 4

Assist allows another player to pay for any amount of generic mana. If an effect changes the cost of the spell, the amount that player may pay will be more or less than the amount in the spell’s reminder text.

Magma Hellion : règles, fonctionnement et interactions (MTG)

Magma Hellion is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Assist (Another player can pay up to of this spell's cost.) Trample, haste

How it works : Magma Hellion

Original English text — Wizards of the Coast / Scryfall

#1June 8, 2018· Wizards of the Coast
You may reveal a card with assist and discuss how to pay for it before you begin to cast the spell. Targets are chosen for that spell before you choose another player to help you pay for it and before that player has committed any mana to doing so.
No other card is named in this ruling.
#2June 8, 2018· Wizards of the Coast
Only the generic mana portion of a spell’s cost can be paid with assist. Any colored mana costs must be paid by the spell’s controller, even if the assisting player paid mana of that color.
No other card is named in this ruling.
#3June 8, 2018· Wizards of the Coast
If an effect allows a player to cast a spell spending mana “as though it were mana of any color” or “of any type,” that player must still pay for the colored mana in that spell’s total cost. That cost doesn’t become generic.
No other card is named in this ruling.
#4June 8, 2018· Wizards of the Coast
Assist allows another player to pay for any amount of generic mana. If an effect changes the cost of the spell, the amount that player may pay will be more or less than the amount in the spell’s reminder text.
No other card is named in this ruling.