I spent some time messing with Halo Spartan Assault.
It's a fun twin stick shooter. There are 30 missions according to the game and at each mission, you have an opportunity to get a bronze star, a silver star, or a gold star. It's a score based game so what star you get is dependent on what score you get. You also get more XP if you get a silver star vs. a bronze star. There are objectives you need to reach at each mission but most of the game is just shooting enemies and hiding while recharging. The game rewards you for being very aggressive: just run into a place and shoot a bunch of shit up. If you're worried about dying, there are numerous things to hide up against. The best way to rack up a high score is pure aggression: you get more points based off of how many enemies you kill in a short period of time. Launching grenades isn't a bad idea either as you usually get a few per stage.
The biggest flaw is that it is an IAP trap. So anyway, I decided to grind a few levels, rack up higher scores, and try to get some XP. There was a booster you could purchase that would give you more points and more XP. So I got that. Well, it turns out, the upgrades you can purchase with your XP are only applicable for that mission. So that XP/Score booster I got only applied to that one mission. However for 99 cents, I could buy what amounted to four booster upgrades.
The game is $14.99 but unless you want to grind a ton of stages to get an upgrade for a single mission, you're probably going to have to shell out a few dollars for credits, which will allow you to buy upgrades. I suppose you can probably complete the game without any kind of upgrades but if you want those gold stars, good luck. I think the game is fun but I suspect it lasts maybe 3-4 hours, depending on how hard you want to try to get the gold star rankings for each mission.