Primordial Hydra

[rnf73trumpet]

If my friend uses a primordial hydra where the strength and toughness double each turn can I use my Boros Reckoned on it?  If he attacks me when it's at 64/64 and I block with a Boros reckoned would it be a stalemate?

[Gorzo]

No, I'm afraid not.  First off, because {Primordial Hydra} gains trample once it gets big, it would only deal 3 damage to a {Boros Reckoner} blocking it. The rest would be dealt to you, killing you before the reckoner's damage trigger can even resolve.

[rnf73trumpet]

So my best bet is to use Act of Treason, Pacify, oblivion ring or other such cards?

[Gorzo]

Pretty much. Removing it is your best bet. Or trickery like {act of treason} or even a well-timed {fog} can bait a hydra-using opponent into a loss

[rnf73trumpet]

Yeah my friend bought that card and it's a pain to deal with.  Now what if I use  {Fog Bank}?

[Dudecore]

Yeah my friend bought that card and it's a pain to deal with.  Now what if I use  {Fog Bank}?

Damage prevention effects are only taken into consideration when damage is being dealt, not when it's assigned. A creature with trample has to assign at least 2 damage to the {Fog Bank}, and the rest can trample over to the player/planeswalker it's attacking. When the damage is dealt, {Fog Bank}'s ability kicks in and prevents the damage to it, but the damage assigned to the player/planeswalker is unaffected.

Same for indestructible creatures. A trample creature needs to only assign 1 point of damage to {Stuffy Doll}, because 1 damage is lethal - until prevention effects are taken into account.