Lizardmen are good at this point level imo.
Salamanders are good versus both HE and empire, due to their 20x20mm toughness 3 infantry units.
Cowboys are good.
Note that pairing up with Delfs makes for a desperate alliance, this means that you have to share the power dice generated from winds of magic as evenly as possible. This makes it very difficult to get off big spells, and means that your magic phase isn't going to be as effective. You might consider leaving the Slann at home and bring an old blood instead.
If you bring an old blood you can play around with various kit-outs. I think you will be well off with the classic CO+GW+AoD+dawnstone setup, but look out sirs are essential vs empire cannons, so you might consider having him on foot unless you are running cavalry. Simply dropping the cold one leaves him at 2+ rerollable armour save, 4+ ward and 5 str 7 non-magical attacks. In this case the mundane weapon that a great weapon is will actually serve you better than a magical one, because of the banner of the world dragon, and that neither helfs nor empire has ethereal units.
I don't know a whole lot about high elves, but I can give you a few pointers about the empire:
Are you playing open lists? Beware of van horstmanns speculum. A 40 point enchanted item that allows you to swap strength, toughness, weapon skill, initiative and attacks value with your opponent in a duel. If this catches you unawares, your saurus lord/hero may sit in a duel with ws 4/5, s4, t4, i4/5 and a 2/3, facing a human opponent with a saurus character stat-line.
Hellblaster volleyguns can ruin your day, 3 artillery dice worth of str 5 AP shots, firing at BS4 if the have an engineer attached. The best solution to this I find is to screen your juicy targets with skink skirmishers, providing hard cover for your important targets. The hellblaster volleygun only has a range of 24", something to consider aswell.
Note that warrior priest/arch lector buffs (hatred and prayers) do not confer to warhorses or demigryphs. RAW say that they do, but the FAQ say that they don't. This is important if your opponent is unaware of the FAQ.
I think you are at a disadvantage because of the fact that you are forced to split power dice between you, this will limit your flexibility in the magic phase a lot, and it forces both of you to bring casters. Maybe play around this by going (relatively) light on magic and play the other phases instead.
Thats all I can think of ATM, good luck!