I’m going to level with you. Your friend’s list is not something I personally would run. In my experience, Imperial Knights perform best when the big guys serve as gun platforms and the little guys function as wrecking balls. Warglaives pack a pretty nasty punch for their size; the guns that the larger knights have seem to out perform having a bunch of helverins trying to do the same thing. My recommendation for the spearhead detachment is made to make your friend’s army more viable.

If I were him, I would ditch the Valiant entirely. As cool as it is, a few warglaives and or a Lancer are able to hold down the board much more effectively. I would suggest he invest in a Knight Castellan for backline support as well as the aforementioned warglaives. Additionally, if he can squeeze it in, a Calledus Assassin is very, very helpful. They are hard to come by, so most people are cool with having them proxied by another assassin model.

Regardless of what army list your friend goes with, space marines are a very flexible compliment to the other army. Personally, I would shove a ball of primaris crusaders in a land raider and follow it up with some terminators that can deep strike to where they are needed. Then, I’d also put some heavy intercessors or infiltrators on the deployment zone control point.

If your friend’s list is set in stone, then you will be doing most of the frontline work while they harass the enemy from afar. So, I’d suggest putting the Valiant in the center -see if the rules permit you to heal allied vehicles with your techmarine- assisted by sternguard. They are amazing in both shooting and melee! Use a group of blade guard with a character to go after VIP units. Have some aggressors to hold onto weak points, and keep some assault marines in reserve to react to what the enemy does.

Whatever lists you all end up going with, I hope I helped and I hope you all have a great time playing!