If you plan on taking the Trygon, take it as a Trygon Prime. The extra dakka and shadow in the warp is a godsend against something like knights. Between that and the winged Tyrant.... dunno. Tempted to say the Tyrant as it'll be harder for him to attack or charge, and it packs more firepower (albeit at a cost of durability). Don't get cocky and go for vector strikes, as the odds are you'll be winged and grounded within charge range.
Depending on what he does, you may find his list actually gets shootier - the ability of purifiers to double up on heavy weapons shouldn't be overestimated. Now normally a million psycannons on foot could be worse - but you, of course, have to walk squarely into range to do anything. The key thing will be protecting the Doom. The achiles heel of purifiers is that they're still just space marines - but for anything up to twice the cost. This means that any firepower optimised to murderize space marines should in theory have a field day.
Oh - one suggestion; if he's going to be having few big, well-armed squads, consider a handful of spore mines. Nothing will irk him like having to waste multiple psycannons on a single spore mine.