You use it before the game begins. You’ll put Ultra Magnus Armor
onto Major Ultra Magnus, then shuffle the rest of your deck and draw 3 cards for your opening hand.
Let's say I have Major Ultra Magnus in bot mode and Private Firedrive
on the field, with Firedrive having 1 health remaining. If I attack with Ultra Magnus and trigger his bot mode ability, which causes Firedrive to die and I put the Duo-Charge Electrostatic Photon Cannon
onto Ultra Magnus, what happens? Does Ultra Magnus get the +3 attack for this battle? Am I able to scrap cards from my hand to increase said attack?
Major Ultra Magnus will get the +3 attack, but since the long-name weapon was attached after he began his attack, the ability will not trigger and you will not be able to scrap any cards to add to your attack.
If I have Major Ultra Magnus with 16 Damage on him and my opponent has Nemesis Prime
with 15 Damage on him and I Attack Optimus Prime - Battlefield Legend
, causing my bot mode skill to trigger, does this cause Major Ultra Magnus to be KO’d and Nemesis Prime to be KO’d therefore no combat with Optimus?
That is correct.
Before the game starts, find their related card, shuffle, then draw three cards for your opening hand.