With your country set to Tyr, 360 turns, proper Troops/Weapons+Mil Strat Tech/Readiness vs their Def, you can achieve the GS kill. IF they are not actively playing.
Think it took like 220 turns to kill Ninjas farm. But thats with Chem rockets, so Warfare vs no/low SDI is helpful
Yes, killing is best suited for the Tyr(anny) country, as it only requires 1 turn per attack.
Prep for GS: Plenty of turns, enough food and cash to allow you to spend 200 or more turns attacking without stopping except to maybe sell food on BM. $$ for buying up more troops if necessary. Enough oil. Then, the critical phase of when to strike. If you know they're active do NOT attack. Even a blitz can be foiled. Signs they are possibly active: NW goes UP {could be standing order, so its a gamble}.
If you suspect they were on, give it about 30 mins or so, then op them. If they're at 0 turns or very few, you've got an excellent window. Even if they sign in the most they can do is buy more troops and stall you. Cant rebuild pop w/o turns.
Spy,cause dissension to reduce troop levels if they are significant, it will reduce your oil and troop cost. Do your final calcs of your troop #, weapons tech, any mil strat tech, vs their troops and tech, plus their govt bonus if they are dict or rep. Launch your GS. Watch your readiness. Most times you can still break when your below 35-40% readiness, but its sinful to waste turns, so now you switch to missles. If he has NO SDI, launch Chem rockets. If he does, lead with cruise, then launch chems. do not nuke, SDI only goes up. When chems are gone, switch to Stir rebellions. Each turn will reduce his pop, while increasing your readiness. also, it can be a good idea to do a few bioterrors, just in case he DOES sign in. Will kill himself trying to repop. When your readiness is back to 95 or 100%, start hitting again. Do an occasional spy op to be sure he isnt buying troops, or spending turns. If you start bouncing or his civ casualties start INCREASING, it means he may be online. Rinse and repeat until dead.