I've recently picked up the Heliod Collected Company deck and I'm not sure how the opponent can interact with the combo. Obviously counter spells will prevent me from playing the cards and hand disruption can take the cards before I play them. But if the situation is that I've got Conclave Mentor and Heliod, Sun-Crowned on the battlefield and turn 4 I have four mana and a Walking Ballista in hand, can a kill spell stop me comboing off? (I play Walking Ballista for 1 with 2 mana, add a counter with Conclave Mentor, give Walking Ballista life link from Heliod for 2 mana, ping opponent for 1, add two more counters on Walking Ballista and continue until the opponents life is zero).
Sure, they can:
- kill either Conclave Mentor or Heliod, Sun-Crowned as soon as you play Walking Ballista (i.e. put it on the stack, before it resolves and enters the battlefield)
- kill Heliod or the Ballista in response to you activating Heliod's last ability (if they kill Heliod, the Ballista will gain lifelink, but it won't get more counters)
- kill the Ballista after you activate its last ability for the first time; you can activate its ability again in response to that, but after paying the cost of the ability it won't have any +1/+1 counters on it and will die from state based effects, unless you have an anthem-like effect in play. In that case, you would be able to activate the Ballista's ability an arbitrary number of times before the kill spell resolves.
The real question you're asking here, is how many kill spells will it take to stop this combo. There are spells where the answer will be one, the spells that have split second:
- Krosan Grip - Walking Ballista (WB) is an artifact, Krosan Grip is a Naturalize with split second, preventing you from continuing the combo until it has resolved.
- Sudden Shock - If WB has up to 2 counters doing 2 damage to it with split second will kill it before you can combo.
- Sudden Death - If WB has up to 4 counters giving it -4/-4 with split second will kill it before it can combo.
- Molten Disaster - X cost split second damage spell, just pay X equal to WB's toughness to kill it - though a warning, this can kill you too if your life is low enough since it hits all players too.
- Word of Seizing - Take control of one of the creatures in the combo. If you take control of WB you can remove the counters yourself and without an effect on your battlefield to replace them, it will die.
There are other split second spells that will temporarily stop the combo, Sudden Spoiling will remove the abilities that the combo relies on, but it can be used next turn.
You can also use a spell like Erase, Deicide, or Swords to Ploughshares (If Heliod(H) is a creature) to exile H before WB resolves, stopping the combo before it starts. WB can also just be countered, stopping the combo from even starting.
But there is some pretty simple math for how many kill spells you need to stop the combo if you let it get started in the first place. For every 2 mana* you have before casting WB, you either can get 1 more WB counter or activate H once. If you spend more mana on casting WB, it takes less kill spells to kill it before it gets lifelink, if you save more mana to use on H, it takes less kill spells to kill WB while you're taking off counters. This means they need the most kill spells if you spend the same amount of mana on WB as you save for H, so for every 4 mana you have, (1 use of H and 1 WB counter) they need 1 kill spell to stop your combo.
*Given an effect like Conclave Mentor or Hardened Scales to put an additional counter on WB or 2 more starting mana into WB. Also given no counter doubling effects like Doubling Season or Primal Vigor which cut the mana to counters rate in half making WB mana twice as efficient.