3* Shoretrooper or 7* Royal Guard for Empire Arena Squad?

I find RG to be quite useful in some cases to save Emperor from an iminent death and to be an extra Stun/Speed Down.

It depends really on who you face in arena. If you need a taunt at start to avoid the AI to kill Emperor before his first move, then Shore. At 3* he is good as long as you gear him enough to give him protection, and fill him with a lot of extra health+protection on bonus sets and secondaries.

RG is becoming "obsolete" for arena with the newer tanks, but he is still good IMO in other game modes besides arena like AAT Raid P3 or GW.

There is no argument out of who is better between RG and Shore in terms of surviving against wiggs teams. Shore allows your empire team to survive against wiggs OHKO so you can land your debuffs, and then kill them quickly. I sit on a wiggs heavy shard with a few droid teams, doesn't sound much different to yours, and my EP, Vader, TFP, Shore, RG team beats wiggs teams easily, so I rank at least top 10 everyday. My Shore is only 3* gear 9 (was gear 8 yesterday) and he still works. The Star count doesn't make much difference, use Shore along with your imperials and they will reward you nicely.