You can use "Prime Material Dragon" to gain 600 life points for each monster instead of taking damage: just make it one of the monsters Special Summoned or already on the field first before the damage is inflicted on resolution of this card's effect.
After this card has resolved, if you have a spare Monster Slot on your field activate "Damage Mage" to Special Summon it to the field and regain all the Life Point damage you took.
Use this card with effects that will negate the damage, such as "One Day of Peace".
You can use "Mystic Tomato" to attack with and then Special Summon more of the same monster card, before finally summoning a Gagaga monster with the final "Mystic Tomato" to make the final suicide attack. Then activate this card and you will get multiple "Mystic Tomato" monsters back on the field and your Gagaga monster all are ready for an Xyz Summon.
This tip also works defensively when your opponent is the one attacking your monsters. Just activate this card when they've declared their End Phase to use what you Special Summon on your next turn.