I've effectively used the flag lock strat a few times.  Run in, grab a portal flag, lock it, teleport out.  Really confuses the enemy, and makes them waste a lot of time.

A tip for getting better: don't use potions, and use teleport scrolls as infrequently as possible.

Honestly, you don't need them, and if you learn to play without having potions on hand then you'll be better for it in the long run.  You'll also have more money to get items you really need.

Regen / +stats items > potions.

Have to say flag locks too. Just played a few games where they were pretty great. Lategame, when you need that extra push for victory, it's just so useful to be able to lock a portal flag and go cap another.

Those scrying things can be useful too. They help defend important flags, and can help against Regulus' mines. They're so cheap that there's no reason not to grab some if your consumables aren't full.

Imo potions aren't necessary, but they can be great. I tend to keep one cheap combat potion on me early, if I got spare gold. It can even help get a kill, as enemy DG stays to fight you thinking you're weak. Kinda risky, but often worth it. One potion isn't that expensive, and if it saves having to run to crystal even once, it's been worth it. But potions are second priority, scrolls and regen items are more important.

I always buy a wand of speed as soon as I can afford it. It's very handy and offers incredible value for money.