Idea 1: 3x squads of 20 man Krieg, with a command squad and Primaris Psyker attached (27 man brick total). The goal is to put huge piles of resilient guys on three objectives, while the Knight (ideally) sits on the home objective. 2 hydras and a leman russ commander loaded with [hazardous] weapons charging down the center, using the Combined Arms detachment.
Infantry get [lethal hits] versus infantry, the hydras are a cheap pinch threat against unlucky enemy elites and anything that can fly. The Leman Russ Commander is kind of a win-win, either the enemy focuses on my knight which leaves the LR's wounds to absorb hazardous damage; or the enemy focuses on the LR, leaving my knight alone longer while possibly triggering an out of phase blast of plasma thanks to Death Befitting of an Officer.
Cons: the knight is basically the only anti-armor/anti-big monster in this set up, since the LR is not taking a las cannon or multi meltas or anything. [LETHAL HITS] plasma cannons might work in a pinch but it's definitely not a reliable solution. This is more or less the extreme ends of one huge guy that I'm hoping lives long enough to draw too much fire away from the infantry swarms.
Idea 2: Thousand Sons, using the Rubricae detachment. Two squads of 10 rubric Marines that get [infiltrators] from the detachment enhancement. The detachment enhancement goes on a sorcerer in terminator armor, leading one minimum size squad of rubric terminators. 2x sets of 2x sekhetar robots gives me 2 more infiltrators. Not sure how to finish the list, a chaos predator and mutilex vortex beast perhaps. The terminators would be kept in reserve for a deep strike, allowing them + the knight to move reactively based on how the opponent tries to deal with ~4 units (24 models) infiltrating to immediately contest mid board.
Idea 3: chaos daemons, using flamers for aggressive chaff removal and beasts of nurgle to force the opponent into really committing to removing one thing every turn. This seems like the most dickish option overall, although it probably lacks the OC or strength to genuinely compete against any SM/CSM.
Idea 4: "custodes" using Null Maiden's Vigil. Maximum number of witchseekers to force battle shock tests, the rest of the sisters would just kinda be cheap chaff.
Idea 5: "custodes" using Solar Spearhead. Send some dreadnoughts in with one calladus.
Thousand Sons is what I'm most likely to do, because that's what I have most of the requisite models for. Similar to the Krieg, they are (relatively) hard to fully remove, although without the elasticity of the guard constantly bringing back models and without the ability to continue contesting objectives while battle shocked.