Take your standard Leman Russ or two, maybe a Basilisk, but try to model it like a WWI artillery gun. Lots of infantry, probably want to go with lots of heavy stubbers or heavy bolters as machine guns were the major killers during WWI. Rough Riders will help for maneuverability, and horses were still around (in limited numbers), I believe.
Probably no storm troopers. I don't think troops were that specialized yet. I think transports should be kept to a minimum, too. I don't think they had too many of those.
However, I don't know guard that well, and I have never been to a GT. I went to a RTT, and didn't do that well. The ones who beat my team were ones that basically had a plan and stuck to it. Each unit had a purpose, and they utilized them to full effect.
Sorry I'm not of more help...
Put up a list of what you think you should have, and I'm sure others will be able to comment effectively on it.
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