Well you see Flag has a decent AP scaling, is all but impossible to dodge, you don't have to worry about minions, and it gives you passive 25% AS and a 25% AS boost when down if maxed meaning that your lack of AD isn't a huge deal since you can win trades with the raw AS and base AD.
In some matchups I run thunderlords since when they get low you can all in with Ult base damage. Something like Jayce is a better all in though so I run DFT and poke him out from behind minions. Mid-Late he will start to fall off but you should have tower by then due to Flag AS boost. You can then group and poke with E and let your team burn objectives very fast with the AOE AS.
I always start corrupting potion into a ludens. If you are going for the splitpush or have the all in advantage a Lich bane is really funny since E > Q > Passive + Lich damage is quite something. You also have a small shield with a slot attached to avoid ganks and a bit of poke.
If you can't all in or splitpush and want to group get a morello for the CDR so you can have permaflags, granting your team a constant 25% AS boost. The flag plus your passive lets you AA clear waves quite quickly.
Early game your poke should be Q > E since you don't want to go in and just want to harass them. Be careful about Qing near E though, it has a really big grab range and you generally don't want to go in unless you can kill.