Tyrant looks good, would suggest toxin sac's to make the dev's str 5.
Warriors w/death are horrible go ahead and drop them.
Devilgaunts, i'd split the squads into 3 squads of 10, mainly because you dont need the troop slots in a small game and you dont want anything hitting CC with 300 points of gaunts that cant fight for beans. loosing 10 would be much less and it allows you to split up your firepower. 60 shots for instance is overkill against most GEQ squads. The problem is you dont have anything threatening in cc to prevent anyone from just moving forward 6'' and rapidfiring.
Hormogaunts, well you really done have any cc support for these guys and i see them probably getting shot to death, especially as they are your only unit that will ever be in rapidfire range, i'd consider just swaping them for more devgaunts.
Carnifex, solid guessing your running a 2tld,+bs+w setup. Kinda slow though.
Well i'd run devilwarriors tbh, 35 points each and they are pretty solid against just about anything.
Probably be a good idea to use the aditional hq for the nids and grab another devil tyrant.
also since you have a fast attack slot 2xacid mines, 24 points and has a decent chance of popping a tank when they come down.
What you really have to consider is that gaunts will never win in a shootout against marines.
SM 10 man tac squad = 4.5 gaunt kills and 9 in rapidfire
Were as an equal 150 points of devilgaunts = 2.25 marine kills.
What you have to consider is that against marines they generally will get the first turn of shooting against your gaunts. Just because of the range of the guns.
Example:
Your devilgaunts are say 24.1''+' away, so you move them forward and cant shoot. Marines in turn openfire on your gaunts from 24'' away and kill 4.5 of them. So your really looking at an uphill battle here. Because you move forward and pop off your 20 s3 devo shots and cause about 1 wound then the marines move forward and just rapidfire you.
My guess though is that people are going to focus on the necrons exclusively just to force the phaseout. (this is what i would do anyways) This basically means your cousin wont have to much fun as people are "picking on him".
It would probably be more advisable for you to run something genestealer heavy.
Tyrant w/Wings,WF,2TLD,+Str
Lictor x1
Genestealers x8
Genestealers x8
Genestealers x8
Zoanthropes w/ warpblast x3 and synapse x1
Devilfex
Add in some spinegaunts or rippers to fill out the remainder points.
Suddenly your units become a much higher priority, and much more dangerous. Quite a bit more antitank than previously so that should help.