So one of the old combat tricks I used to like to do back when combat used the stack was attack with Volrath's Shapeshifter, get her blocked, discard into Phyrexian Dreadnought, get trample damage assigned, and shift to Phage the Untouchable before it was dealt. Is there any clever way to still do this?
While I'm at it, I'm pretty sure Phage with trample and lethal combat damage will kill the opponent and Phage simultaneously. Would like rules reasons for this.
In general, no, if what you seek to do is have Volrath's Shapeshifter assign combat damage while it does not have Phage's last ability, but deal combat damage while it does. (Suggesting specific cards that would make it possible would be outside the scope of the forum.) That is because under current rules, no player gets priority to cast spells or activate abilities between the time combat damage is assigned and the time it's dealt (C.R. 510.2).
For your second question:
If enough combat damage is dealt to a player to bring that player's life total to 0 or less, and if enough combat damage is dealt to a creature (such as Phage) that that creature has lethal damage, then simultaneously the player loses the game and the creature is destroyed (C.R. 610.3, 117.5, 704.5a, 704.5f, 704.3). This much is true.
However, this is not generally true if one but not the other applies. For example:
If a creature deals combat damage to a player while it has an ability of the form "Whenever [this creature] deals combat damage to a player..." (such as Phage's last ability or that of Ninja of the Deep Hours), that ability will trigger (C.R. 602.3).
Then the active player would get priority (C.R. 610.3), but first state-based actions are checked (C.R. 117.5), including the following:
— If one or more players' life totals are 0 or less, those players lose the game (C.R. 704.5a). And simultaneously (C.R. 704.3)...
— If one or more creatures have lethal damage, those creatures are destroyed (such as Phage if it has been dealt lethal damage, but this won't affect any of its abilities waiting to go on the stack [C.R. 113.7a]) (C.R. 704.5f).
Then triggered abilities that have just triggered go on the stack, such as the ability mentioned earlier (C.R. 117.5).
Then more state-based actions are checked and more triggered abilities put on the stack as necessary, until nothing further happens (C.R. 117.5).
Then the active player gets priority (C.R. 117.5).
Assume all players now pass. Then the topmost ability on the stack resolves (which will be the ability mentioned in step 1 if no other abilities were put on the stack in this process), and in the case of Phage's last ability, the player it refers to will lose the game (C.R. 117.4, 608.2).
While I'm at it, I'm pretty sure Phage with trample and lethal combat damage will kill the opponent and Phage simultaneously. Would like rules reasons for this.
In general, no, if what you seek to do is have Volrath's Shapeshifter assign combat damage while it does not have Phage's last ability, but deal combat damage while it does. (Suggesting specific cards that would make it possible would be outside the scope of the forum.) That is because under current rules, no player gets priority to cast spells or activate abilities between the time combat damage is assigned and the time it's dealt (C.R. 510.2).
For your second question:
If enough combat damage is dealt to a player to bring that player's life total to 0 or less, and if enough combat damage is dealt to a creature (such as Phage) that that creature has lethal damage, then simultaneously the player loses the game and the creature is destroyed (C.R. 610.3, 117.5, 704.5a, 704.5f, 704.3). This much is true.
However, this is not generally true if one but not the other applies. For example: