At 3750 you have 937.5 points of lords, which means you can get two tooled up slanns easily.
Go light/death or light/shadow to complement each lore i.e buffs and debuffs
Then take multiple tooled up scar vets for the hard fighting in the trenches, a couple of large blocks of troops and then lots of skirmishers and terradons and such if you say your Bret ally is bring Cavalary and war machines.
Stegs might be useful but Bret Cav does a similar job, and you will need some thing to break ranks/grind out units which both stegs and cav arent good at, I would say take a large saurus and large skrox block and use them to do the hard fighiting while brets do the hard hit into elite units, while skirmishers acts as pest/redirector/war machine hunters and the dual slann rules the magic phase
"one light, one life and put them in a very big templeguard unit" the wording here suggests both slann in the same unit of TG, if that is what is meant by this, you cant do that, only one slann per TG unit